Add typing.ClassVar (fixes #888)

Author: miedzinski

stdlib/3/typing.pyi

@@ -16,6 +16,9 @@ Type = object()
 _promote = object()
 no_type_check = object()
 
+if sys.version_info >= (3, 5):
+    ClassVar = object()
+
 class GenericMeta(type): ...
 
 # Type aliases and type constructors