Add stubs for UserList, UserString, UserDict.

stdlib/2.7/collections.pyi

@@ -2,16 +2,13 @@
 
 # Based on http://docs.python.org/2.7/library/collections.html
 
-# TODO UserDict
-# TODO UserList
-# TODO UserString
 # TODO more abstract base classes (interfaces in mypy)
 
 # NOTE: These are incomplete!
 
 from typing import (
     Dict, Generic, TypeVar, Iterable, Tuple, Callable, Mapping, overload, Iterator, Sized,
-    Optional, List, Set, Sequence, Union, Reversible
+    Optional, List, Set, Sequence, Union, Reversible, MutableMapping, MutableSequence
 )
 import typing
 
@@ -22,8 +19,6 @@ _VT = TypeVar('_VT')
 # namedtuple is special-cased in the type checker; the initializer is ignored.
 namedtuple = object()
 
-MutableMapping = typing.MutableMapping
-
 class deque(Sized, Iterable[_T], Reversible[_T], Generic[_T]):
     def __init__(self, iterable: Iterable[_T] = ...,
                  maxlen: int = ...) -> None: ...