Ongoing code maintenance and quality tasks

[Sorin Sbarnea]

When we are blocked on more important tasks, or we need a mental break from heavier activities, please pick one of the ones listed below:

• increasing code coverage
• reduce linter rule exclusions (flake8 or pylint)
• improve typing (mypy), aiming to enable --strict mode
• review pending pull requests and fix, especially ones from pre-commit auto-update or dependabot ones
• add missing docstrings with params and returns

Keep the PRs small!