Add pyupgrade to pre-commit hooks

[hwine]

pyupgrade can help with moving the code base to utilize newer python features, such as f-strings. I roughly works like black, in that it will modify the files, allowing easy inspection of the changes.

Current plan:

• introduce & pin to --py38-plus
• examine scope of changes to decide if "flag day" approach is needed.
• land it

Notes:

• Updating the python version for this plugin will never be a blocker to upgrading the supported version.
• Any maintenance of the python version will be delegated to the "bump version" solution