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.

Python/fileutils.c

@@ -1842,14 +1842,6 @@ Py_fopen(PyObject *path, const char *mode)
 }
 
 
-// Deprecated alias to Py_fopen() kept for backward compatibility
-FILE*
-_Py_fopen_obj(PyObject *path, const char *mode)
-{
-    return Py_fopen(path, mode);
-}
-
-
 // Call fclose().
 //
 // On Windows, files opened by Py_fopen() in the Python DLL must be closed by