bpo-26579: Add object.__getstate__(). (GH-2821)

Copying and pickling instances of subclasses of builtin types
bytearray, set, frozenset, collections.OrderedDict, collections.deque,
weakref.WeakSet, and datetime.tzinfo now copies and pickles instance attributes
implemented as slots.

Include/object.h

@@ -299,6 +299,11 @@ PyAPI_FUNC(void) PyObject_ClearWeakRefs(PyObject *);
 */
 PyAPI_FUNC(PyObject *) PyObject_Dir(PyObject *);
 
+/* Pickle support. */
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(PyObject *) _PyObject_GetState(PyObject *);
+#endif
+
 
 /* Helpers for printing recursive container types */
 PyAPI_FUNC(int) Py_ReprEnter(PyObject *);