Add --strict-optional flag to mypy (#368)

tests/mypy_test.py

@@ -116,6 +116,7 @@ def main():
         if files:
             runs += 1
             flags = ['--python-version', '%d.%d' % (major, minor)]
+            flags.append('--strict-optional')
             sys.argv = ['mypy'] + flags + files
             if args.verbose:
                 print("running", ' '.join(sys.argv))