Run flake8/mypy/radon sequentially

We have a recurring data race in flake8 tasks, probably caused by 'tox -e mypy'
creating and deleting files while flake8 scans them; so we should not run
them at the same time.

While we are at it; also run radon sequentially. flake8 and radon each take less
than 10s, so we should not notice much of a difference.