Fixed for mypy 0.501.

The List and Dict types are no longer implicitly available. They must be
imported from typing.

Type annotations must appear before the doc comment in a function. Also
fix type errors in these functions that weren't detected before.

lib/cretonne/meta/srcgen.py

@@ -11,7 +11,7 @@ import os
 import re
 
 try:
-    from typing import Any  # noqa
+    from typing import Any, List  # noqa
 except ImportError:
     pass