Split header files to move the non-limited API to Include/cpython/:
* Include/warnings.h => Include/cpython/warnings.h
* Include/weakrefobject.h => Include/cpython/weakrefobject.h
Exclude PyWeakref_GET_OBJECT() from the limited C API. It never
worked since the PyWeakReference structure is opaque in the limited C
API.
Move _PyWarnings_Init() and _PyErr_WarnUnawaitedCoroutine() to the
internal C API.