Check all our python scripts with mypy

This is a first attempt at static checking our python scripts, to prepare
adding more type annotations in the future.

- Rework bits to make mypy happy.
- Add a mypy config file to deal with the junit.xml module.
- Check all scripts with mypy in `make check'.
- Mention mypy in the README and in `make help'.
- Add mypy install to the CI files.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>