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 #247 fixing something that hasn't yet been released.

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

• Drop support for
  • Drop support for JupyterHub 0.9 and Python 3.6 #271
  • Resolved by maint: req py36+ and jh 1.5.1+, fix tests, add RELEASE.md, add pre-commit hooks, add dependabot #273
• Add test for JupyterHub 4
  • Resolved by maint: req py36+ and jh 1.5.1+, fix tests, add RELEASE.md, add pre-commit hooks, add dependabot #273
• Debug/resolve test failures
  • Resolved by maint: req py36+ and jh 1.5.1+, fix tests, add RELEASE.md, add pre-commit hooks, add dependabot #273
• Add pre-commit hooks for autoflake, pyupgrade, isort
  • Resolved by maint: req py36+ and jh 1.5.1+, fix tests, add RELEASE.md, add pre-commit hooks, add dependabot #273
• Add RELEASE.md, adopt tbump, let version.py be in package
  • Resolved by maint: req py36+ and jh 1.5.1+, fix tests, add RELEASE.md, add pre-commit hooks, add dependabot #273
• Review PRs
  • Upgrade singleuser.py to JupyterHub 4 #267
  • Change port number for JUPYTERHUB_SERVICE_URL #268
• Make a new release

Optional bonus

• ci: transition to jh-org typical release workflow #269