Add pre-commit setup and enable in in CI

[ulgens]

The repo doesn't have a lint / format setup that works automatically in CI. I'm planning to add one with pre-commit, similar to

• https://github.com/django/birthday20/blob/2228215a3be6717524d6e31e5ac571a5d0273161/.github/workflows/pre-commit.yml
• https://github.com/django/birthday20/blob/2228215a3be6717524d6e31e5ac571a5d0273161/.pre-commit-config.yaml

and enrich this config with pyupgrade and django-upgrade