CI: Pin flake8 < 3.8.0
xref #34150

This handles multiple issues:
- flake8 >= 3.8.1 introduces a new version of pycodestyle which introduces
  extra checking
- flake8-rst is broken with the newest version of flake8
    - See kataev/flake8-rst#22

A follow-up PR handling the first of these is forth-coming