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Can't this code:
 class Sequence(Sized, Iterable, Container):
 # ...
 def __contains__(self, value):
 for v in self:
 if v == value:
 return True
 return False
Be shortened into this: 
 class Sequence(Sized, Iterable, Container):
 # ...
 def __contains__(self, value):
 return any(item == value for value in self)
Which can even fit on one line with a lambda: 
 class Sequence(Sized, Iterable, Container):
 # ...
 __contains__ = lambda self: any(item == value for value in self)

This is slower.
>>> import timeit
>>> class A(list):
... def __contains__(self, value):
... for v in self:
... if v == value:
... return True
... return False
... 
>>> timeit.timeit('500 in x', setup='from __main__ import A; x = A(range(1000))', number=10000)
1.1222619999971357
>>> class B(list):
... def __contains__(self, value):
... return any(v == value for v in self)
... 
>>> timeit.timeit('500 in x', setup='from __main__ import B; x = B(range(1000))', number=10000)
2.05952100000286

Oh. I wonder why `any` is slow like that, you'd figure it's be optimized.

Because in first case there is one iterator, iter(self), and in second case 
there are two iterators: iter(self) and iter((v == value for v in self)).

Thanks for the clarification. Oh well, sad to see the more verbose code win, but I guess that's life.
I tried on PyPy but the difference was even more pronounced, 0.008922450399566156 for the long version and 0.042124665810088044 for the short version.