Update pre-commit hooks

[kurtmckee]

This PR introduces the following changes:

• Fix deprecated pre-commit stage names.

  When pre-commit install is run, it currently throws a deprecation regarding the stage names in the config file. This is now fixed.

• Update the pyupgrade hook to its latest version, and update its target syntax version to Python 3.9+.

• Remove flynt as a pre-commit hook.

  Its functionality is handled by the pyupgrade hook.

• Migrate from local pre-commit hooks to standard, hosted pre-commit hooks.

  This change is necessary because not all git-related tooling runs pre-commit hooks with the same activated virtual environment that the Poetry dev dependencies are installed in.

Taken together, these changes bring the pre-commit hook configuration up-to-date, and make it possible to enable pre-commit.ci for the repo if you choose to do so (and I highly recommend doing so!).

[!NOTE]

I've split the commits up so that it can be reviewed commit-by-commit. This isolates the manual and automatic changes so that they can be reviewed in smaller chunks.