Add flynt package to linter

[altheaden]

I'm updating the linting procedure to include flynt, which is a package that automatically searches for and replaces older-style format strings with f-strings (in most cases). This is a change we made a while ago in polaris (E3SM-Project/polaris#74).

Checklist

• Testing comment in the PR documents testing used to verify the changes

[altheaden]

Testing

I created the mache dev environment and verified that flynt was installed. Then, I created a python file with an old-style format string and ran pre-commit on that file, and it flagged and automatically fixed the format string as expected.

[xylar]

@altheaden, looks great!