Upgrade syntax to Python 3.7

[nsoranzo]

using ack --type=python -f | grep -v '^tools/' | xargs pyupgrade --py37-plus

Also more refactoring of Python files in packages/ .

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[bgruening]

Cool, but I guess we should wait until @jmchilton work has landed?

[nsoranzo]

Which one? This is very small compared to the black PR.

[jmchilton]

I have reservations about this but my open PRs are a non-issue here, feel free to merge.

[nsoranzo]

Rebased, and I've converted (almost) all format strings to f-strings. Ready for a re-review.

[nsoranzo]

Thanks for the reviews @bgruening @ic4f !