Enforce cycle checking.

Author: tseaver

pylintrc_default

@@ -24,7 +24,7 @@ ignore = datastore_v1_pb2.py
 [MESSAGES CONTROL]
 disable = I, protected-access, maybe-no-member, no-member,
           redefined-builtin, star-args, missing-format-attribute,
-          similarities, cyclic-import, arguments-differ,
+          similarities, arguments-differ,