Add pre-commit & make webp dependency optional

[pajowu]

All changes in the pre-commit commit are automatically created by black and should be safe except for three:

• https://github.com/okfde/django-filingcabinet/commit/c2b73e2b80f827e2476678bcc738aceb5c9a1175#diff-56e2272179d57ef3d6a4e126432302a140f2511fe8b845c9447b585558bbfe27R210
• https://github.com/okfde/django-filingcabinet/commit/c2b73e2b80f827e2476678bcc738aceb5c9a1175#diff-56e2272179d57ef3d6a4e126432302a140f2511fe8b845c9447b585558bbfe27R357
• https://github.com/okfde/django-filingcabinet/commit/c2b73e2b80f827e2476678bcc738aceb5c9a1175#diff-b80182c4bc017f5f70fc38e25bd55435a08be07c569b9c2f46d8b2a9dbeda73eR157

I think those should be safe as well. The first two are to fix a B007, the third one to fix a B008