[CI] Force usage of f-strings

[mx-psi]

What does this PR do?

Add a Python linter to force people to use f-strings as the preferred string formatting option.
Change existing usages to f-strings with the help of flynt, and skip some of the more troublesome refactors.

Motivation

• Ensure consistent string formatting usage across all codebase
• Ensure linters can detect unused or undefined variables (which are detected reliably, unlike dictionary keys)

Additional Notes

I divided the code rewriting into the automatic and manual parts; please look more carefully into the manual changes.

Possible Drawbacks / Trade-offs

1. The automatic change is pretty simple and the flynt tool seems to be widely used, so I wouldn't worry that much about these.
2. We may find that using format is necessary in some cases (e.g. when formatting raw strings or repeating the same arg several times). We can skip the linter in those places but I would expect them to be rare.
3. This bumps the Python requirement for tooling to Python 3.7 or later (2018); I think that should be fine.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• The appropriate team/.. label has been applied, if known.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the config template has been updated.

[mx-psi]

This was missing the build tags and exec opts

[alai97]

Looks good for docs!