bpo-45459: Add pytypedefs.h header file (GH-31527)

Move forward declarations of Python C API types to a new pytypedefs.h
header file to solve interdependency issues between header files.

pytypedefs.h contains forward declarations of the following types:

* PyCodeObject
* PyFrameObject
* PyGetSetDef
* PyInterpreterState
* PyLongObject
* PyMemberDef
* PyMethodDef
* PyModuleDef
* PyObject
* PyThreadState
* PyTypeObject