DESCRIPTION Title of Invention METHOD FOR MANUFACTURING SHAPED PRODUCT BY LOW-PRESSURE MOLDING Technical Field The present invention relates to a method of manufacturing a shaped product including a thermoplastic resin and carbon fibers. More specifically, the present invention relates to a method for manufacturing a shaped product including a thermoplastic resin and carbon fibers by a press molding method at a low pressure, and the shaped product obtained from the method. Background Art A fiber-reiforced composite material, especially a composite material in which carbon fibers are used as reinforcing fibers, is expected to be applied to applications of a vehicle requiring to be light-weighted due to its high specific strength and specific rigidity. In particular, in a case where a matrix resin is thermoplastic, the material is promising from the standpoint of productivity and recyclability. Patent Document 1 discloses a method including, impregnating carbon fibers having a specific fiber length with a resin, forming a preform in a sandwich structure by using a composite material precursor (prepreg) in which two single carbon fibers intersect within a specific range, and then press-molding the preform. Here, as a specific example, there is provided a sheet-making method which employs a wet method in order to intersect two single carbon fibers within a specific range. However, in many cases, such a method may require a drying process or the like, thereby complicating a manufacturing process. Further, it is difficult to form a prepreg with a relatively large thickness. Then, a large number of layered sheets are required at the time of molding, thereby complicating a molding process. Patent Document 2 discloses a sheet material for a fiber-reinforced composite shaped product in which a plurality of reinforcing fiber bundles are randomly placed in a thermoplastic resin as a matrix. Here, in a state where the reinforcing fiber bundles are pulled and aligned at a predetermined ratio, the thermoplastic resin covers the periphery of the fiber bundles or impregnates into the fiber bundles, and then the fiber bundles are cut to be formed as chopped strands. Further, in Patent Document 2, the sheet material is inserted into a mold and molded by hot press. In this method, since the fiber bundles are cut after being covered with or impregnated with the thermoplastic resin, the cut chopped strands have a certain width and their fiber orientation tends to be close to one direction. Thus, there is a problem in that a high isotropy is hardly achieved even if the chopped strands are uniformly dispersed to obtain a shaped product sheet. Patent Document 1 : Japanese Patent Laid-Open Publication No. 20 1 0-23 5779 Patent Document 2: Japanese Patent Laid-Open Publication H 10-3 1677 1 Summary of Invention A main object of the present invention is to provide a novel method of manufacturing a shaped product including a thermoplastic resin and carbon fibers. Another object of the present invention is to provide a novel method of manufacturing the shaped product at a lower pressure by using a press molding method. A further object of the present invention is to provide a method of easily manufacturing a large-sized shaped product by low-pressure molding. A still further object of the present invention is to provide a shaped product which is good in mechanical isotropy, which may be manufactured by the above manufacturing methods. Other objects and advantages of the present invention will be apparent from the following description. The present inventors have investigated a method of manufacturing a shaped product (CFRTP) including carbon fibers and a thermoplastic resin by a low-pressure molding method excellent in productivity. In particular, they have focused on chopped strands with a specific length and a carbon fiber bundle thereof. As a result, they completed the present invention based on knowledge that it is very important to use a mat base material having the carbon fiber bundle satisfying a specific opening degree. According to the present invention, objects and advantages of the present invention are achieved by: [I] a method of manufacturing a shaped product including carbon fibers and a thermoplastic resin, the method including: (1) a process of preparing a random mat which includes the carbon fibers including carbon fiber bundles with an average fiber length of 5 mm to 100 mm and the thermoplastic resin; 3 (2) a process of impregnating the thermoplastic resin into the carbon fiber bundles by heating and pressurizing the random mat up to a temperature of a melting point or more and less than a decomposition temperature in a case where the thermoplastic resin is crystalline, or up to a temperature of a glass transition temperature or more and less than a decomposition temperature in a case where the thermoplastic resin is amorphous before or after arranging the random mat within a mold; (3) a process of pressurizing and molding the impregnated random mat within the mold at a pressure ranging from 0.1 MPa to 20 MPa; and (4) a process of taking out the obtained shaped product from the mold, in which, the processes (1) to (4) are performed in this order, in the random mat, the carbon fibers have a fiber areal weight ranging fiom 25 g/m2 to 10,000 g/m2, are substantially randomly oriented in-plane direction, and the carbon fiber bundles include carbon fiber bundles (A) having the carbon fibers of not less than a critical number of single fiber defined by following equation (I), and carbon fibers (B) having the carbon fibers of less than the critical number of single fiber, in which a ratio of the carbon fiber bundles (A) is 20 Vol% or more and less than 99 Vol% based on a total amount of the carbon fibers in the random mat. critical number of single fiber = 6001D (1) (in which, D represents an average fiber diameter (pm) of single carbon fiber) Also, the present invention includes the following invention. [2] The method according to [I], wherein the process (1) includes a process (1-1) of cutting and then opening the carbon fibers. [3] The method according to [I] or [2], wherein, in the process (3), the pressure ranges from 0.5 MPa to 10 MPa. [4] The method according to [3], wherein the pressure ranges from 0.5 MPa to 5 MPa. [5] The method according to any one of [I] to [4], wherein the process (2) includes a process (2-1) of impregnating the thermoplastic resin into the carbon fiber bundles by heating and pressurizing the random mat before arranging the random mat within the mold, and the process (3) includes a process (3-1) of arranging the impregnated random mat within the mold, and pressurizing and molding the random mat in the mold controlled to a temperature less than a crystallization temperature in a case where the thermoplastic resin is crystalline, or to a temperature less than a glass transition temperature in a case where the thermoplastic resin is amorphous. [6] The method according to [5], wherein the process (3-1) includes a process (3-2) of 4 clamping the mold at a slide lowering speed of 50 mdsec to 10,000 mdsec before pressurizing the random mat. [7] The method according to any one of [I] to [4], wherein the process (2) includes a process (2-2) of impregnating the thermoplastic resin into the carbon fiber bundles by heating and pressurizing the random mat, due to heating the mold up to the temperature of a melting point or more and less than the decomposition temperature in a case where the thermoplastic resin is crystalline, or up to the temperature of a glass transition temperature or more and less than the decomposition temperature in a case where the thermoplastic resin is amorphous, after arranging the random mat within the mold, and the process (3) is performed after the process (2-2). [8] The method according to any one of [I] to [7], wherein a content ratio of the thermoplastic resin to the carbon fibers in the random mat ranges from 50 parts to 1,000 parts by weight based on 100 parts by weight of the carbon fibers. [9] The method according to any one of [I] to [8], wherein an average number of the fibers (N) in the carbon fiber bundles (A) in the random mat satisfies following equation (2). 0 . 71~04 ID2