Add collections.Container to Python 2/3 (#608)

stdlib/3/collections/__init__.pyi

@@ -13,6 +13,7 @@ from typing import (
 # These are exported.
 # TODO reexport more.
 from typing import (
+    Container as Container,
     MutableMapping as MutableMapping,
     Sequence as Sequence,
     MutableSequence as MutableSequence,