The docstrings (at least in 3.6) say subclasses must override __new__ *or* __init__. However, I think this is wrong too. The following is a correct (if not very useful) implementation of Sequence:

>>> import collections.abc
>>> class MySequence(collections.abc.Sequence):
...     def __getitem__(self, key):
...             raise IndexError(key)
...     def __len__(self):
...             return 0
... 

Other abc docstrings also don't claim that __init__ or __new__ must be implemented. The attached patch fixes the docstrings.