[capi-sig]Some work on reorganizing header files of CPython

Hi,

I working on the Include/ directory of CPython to separate the different levels of API into subdirectories:

Move internal headers to Include/internal/ : https://bugs.python.org/issue35081

Move !Py_LIMITED_API to Include/pycapi/: https://bugs.python.org/issue35134

I also started to use replace macros with static inline functions:

Convert Py_INCREF() and PyObject_INIT() to inlined functions https://bugs.python.org/issue35059

All these changes are connected to my https://pythoncapi.readthedocs.io/ project. It's the very first steps towards a better C API. The most concrete changes are that Py_BUILD_CORE are now in separated header files and so removed from Include/. You have to explicitly include pycode_xxx.h (ex: pycode_accu.h).

Victor