gh-106320: Remove private _PyObject C API (#107159)

Move private _PyObject and private _PyType functions to the internal
C API (pycore_object.h):

* _PyObject_GetMethod()
* _PyObject_IsAbstract()
* _PyObject_NextNotImplemented()
* _PyType_CalculateMetaclass()
* _PyType_GetDocFromInternalDoc()
* _PyType_GetTextSignatureFromInternalDoc()

No longer export these functions.