GitHub Action to lint Python code

[cclauss]

Test results: https://github.com/cclauss/chronam/actions

[acdha]

My concern with this is that the problem isn't really running the linters but someone needing time for major upgrades — more than pyupgrade + django-upgrade are going to handle automatically.

[cclauss]

We can add things to pre-commit over time.

[acdha]

I think that makes sense. For example, we could do a simple pyupgrade pass and then enable that hook so we don't train everyone to think of jobs failing as normal.

Thanks for the work you've been contributing!