gh-128863: Deprecate private C API functions (#128864)

Deprecate private C API functions:

* _PyBytes_Join()
* _PyDict_GetItemStringWithError()
* _PyDict_Pop()
* _PyThreadState_UncheckedGet()
* _PyUnicode_AsString()
* _Py_HashPointer()
* _Py_fopen_obj()

Replace _Py_HashPointer() with Py_HashPointer().

Remove references to deprecated functions.