Enable pre-commit.ci service

[nijel]

To fully utilize https://github.com/python-social-auth/social-core/pull/643 or https://github.com/python-social-auth/social-core/pull/644 we need a pre-commit.ci service enabled. See https://pre-commit.ci/ for more details why it is better to run it there instead of using GitHub Actions.

I've already sent approval request, it can be approved by @omab here:

https://github.com/organizations/python-social-auth/settings/installations