pre-commit: add config and small fixes

[consideRatio]

The JupyterHub github organization has adopted pre-commit and the automation to run pre-commit via pre-commit.ci quite systematically. I've waited to make this PR until getting a few recently active PRs merged, which they now are.

• .pre-commit-config.yaml, .flake8, and settings for pyproject.toml are copied from https://github.com/jupyterhub/jupyterhub-python-repo-template with minor adjustments.
• Closes Consider using black/pre-commit for nicer code #296

Note that all autoformatting is put in the final commit by pre-commit.ci's automation, and all manual changes in previous commits by me.

[consideRatio]

Rebased to handle merge conflicts.

[yuvipanda]

Thanks @consideRatio