General maintenance needed (jh 4 not tested, tests broken, release needed)

[consideRatio]

I saw a forum post indicating that an issue was fixed by using the latest version of this project, and that seems to be related to https://github.com/jupyterhub/batchspawner/pull/247 fixing something that hasn't yet been released.

Overall, this project is in need of some general maintenance:

• [x] Drop support for
  • #271
  • Resolved by #273
• [x] Add test for JupyterHub 4
  • Resolved by #273
• [x] Debug/resolve test failures
  • Resolved by #273
• [x] Add pre-commit hooks for autoflake, pyupgrade, isort
  • Resolved by #273
• [x] Add RELEASE.md, adopt tbump, let version.py be in package
  • Resolved by #273
• [x] Review PRs
  • #267
  • #268
• [ ] Make a new release

Optional bonus

• #269