add missing methods to DictMixin (#2054)

Fixes #1896.

Author: JelleZijlstra

stdlib/2/UserDict.pyi

@@ -17,6 +17,8 @@ class IterableUserDict(UserDict[_KT, _VT], Generic[_KT, _VT]):
 
 class DictMixin(Iterable[_KT], Container[_KT], Sized, Generic[_KT, _VT]):
     def has_key(self, key: _KT) -> bool: ...
+    def __len__(self) -> int: ...
+    def __iter__(self) -> Iterator[_KT]: ...
 
     # From  typing.Mapping[_KT, _VT]
     # (can't inherit because of keys())