bpo-27275: Change popitem() and pop() methods of collections.OrderedDict (GH-27530)

* Unify the C and Python implementations of OrderedDict.popitem().

The C implementation no longer calls ``__getitem__`` and ``__delitem__``
methods of the OrderedDict subclasses.

* Change popitem() and pop() methods of collections.OrderedDict

For consistency with dict both implementations (pure Python and C)
of these methods in OrderedDict no longer call __getitem__ and
__delitem__ methods of the OrderedDict subclasses.

Previously only the Python implementation of popitem() did not
call them.