CI: update pytest setup and show annotations on PRs

[Minnozz]

• Update to action/checkout@v4
• Update to action/setup-python@v5
• Cache pip dependencies
• Merge Python workflows into a single file
• Use .env.example as environment with fewer overrides
• Show pytest failures as annotations on the PR
  • Example: https://github.com/bookwyrm-social/bookwyrm/pull/3277/files (at the bottom of the page)