bpo-31497: Add private helper _PyType_Name(). (#3630)

This function returns the last component of tp_name after a dot.
Returns tp_name itself if it doesn't contain a dot.

Include/object.h

@@ -501,6 +501,7 @@ PyAPI_FUNC(PyObject *) PyType_GenericAlloc(PyTypeObject *, Py_ssize_t);
 PyAPI_FUNC(PyObject *) PyType_GenericNew(PyTypeObject *,
                                                PyObject *, PyObject *);
 #ifndef Py_LIMITED_API
+PyAPI_FUNC(const char *) _PyType_Name(PyTypeObject *);
 PyAPI_FUNC(PyObject *) _PyType_Lookup(PyTypeObject *, PyObject *);
 PyAPI_FUNC(PyObject *) _PyType_LookupId(PyTypeObject *, _Py_Identifier *);
 PyAPI_FUNC(PyObject *) _PyObject_LookupSpecial(PyObject *, _Py_Identifier *);