Add Counter() to __all__.

Lib/collections.py

-__all__ = ['deque', 'defaultdict', 'namedtuple']
+__all__ = ['Counter', 'deque', 'defaultdict', 'namedtuple']
 # For bootstrapping reasons, the collection ABCs are defined in _abcoll.py.
 # They should however be considered an integral part of collections.py.
 from _abcoll import *