I should have added the mixin methods for the Sequence implementation; the more complete demonstration is:

>>> from collections.abc import Sequence, Container, Sized
>>> class MySequence(object):
...     def __contains__(self, item): pass
...     def __len__(self): pass
...     def __iter__(self): pass
...     def __getitem__(self, index): pass
...     def __len__(self): pass
...     def __reversed__(self): pass
...     def index(self, item): pass
...     def count(self, item): pass
... 
>>> issubclass(MySequence, Container)
True
>>> issubclass(MySequence, Sized)
True
>>> issubclass(MySequence, Sequence)
False