Strict mypy - Phase 1

[svartkanin]

This PR is to start the enforcement of strict mypy validation.
The plan is to add more and more files after they have been cleaned up which means that any new changes to such files have to be compliant. This way not all files need to be fixed at once but it can be done gradually.

• Add strict mypy compliant configuration mypy.ini that is validated alongside the existing "looser" checks
• Modify the existing workflows to install the mypy/flake8 version specified in the pyproject.toml file rather than puling the latest as they may diverge in checks