Fix pre-commit hook failures

[kurtmckee]

This PR resolves problems with pre-commit hooks.

• The pre-commit hook versions are very out-of-date. The first commit updates them using pre-commit autoupdate.
• tox -e format disagreed with the pre-commit hook reorder_python_imports. A commit replaces the pre-commit hook with an isort hook so there are no disagreements.
• pyupgrade was targeting Python 3.7 syntax, but Quart requires Python 3.8 or higher. A commit updates pyupgrade to target Python 3.8 syntax.
• pre-commit run --all-files was run twice to catch and fix most issues.
• Flake8 failures were then manually resolved.

The test suite continues to fail due to mypy issues and a missing toml dependency. Merge #254 to resolve the toml failure.

Also, I recommend enabling pre-commit.ci for this repo.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[pgjones]

Thanks I've merged most of this manually.