Even more setting up to run mypy as per #496.

Change-Id: Ib1250ec954c0fecdaed606c41bbbbe00ff7bc1de
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63371
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>