really drop python<=3.7 support

[kloczek]

Summary

Filter all code over pyupgrade --py38-plus.

Tasks

• [ ] Added unit tests
• [ ] Added documentation for new features (where applicable)
• [ ] Added release notes (using reno)
• [ ] Ran test suite and style checks and built documentation (tox)

Further details

N/A

[stephenfin]

Can you run this through pre-commit or tox -e style to address the lint issues and I'll merge?