bpo-35134: Split warnings.h and weakrefobject.h (GH-29042)

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.