mypy: Added Dict, List and Set imports.

Fixed mypy errors associated with the upgrade.

tools/lib/find_add_class.py

@@ -1,7 +1,7 @@
 from __future__ import absolute_import
 from __future__ import print_function
 
-from typing import Set, Tuple
+from typing import List, Set, Tuple
 
 import os
 import re