Develop PR checklist (beyond black, flake8 etc)

[matentzn]

Lessons learnt from @cthoyt so far:

• [ ] Ensured docstrings of all functions I touched are up to date
• [ ] Checked that my coding aligns with the @cthoyt style guide
• [ ] Ensure all my variables have meaningful names
• [ ] Consider keyword-only arguments were appropriate
• [ ] Dont add defaults in to the function definition, default to None and handle defaults inside the function
• [ ] Precompile frequently use regexes
• [ ] Use imperative mood in docstrings and always end with a period

We may want to add this checklist or a reduced or better version as a PR template as well.