Drop support for EOL Python 3.7

[hugovk]

Python 3.7 is EOL this month:

• https://devguide.python.org/versions/
• https://peps.python.org/pep-0537/

Let's drop it. I don't mind if we wait until 2023-06-27, but we're probably close enough?

---

Does dropping a Python warrant a major or minor bump?

I used to do major, but python_requires = >=3.8 ensures the new release only installs for the right group, and I'm now leaning towards a minor, but either is fine with me.

(If we had deprecations to drop, I'd do those at the same time and do major, but we don't.)

A recent poll on the topic favours minor:

[codecov[bot]]

Codecov Report

Merging #551 (d8c7596) into master (f02ac49) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #551   +/-   ##
=======================================
  Coverage   91.40%   91.40%           
=======================================
  Files          28       28           
  Lines        2723     2723           
=======================================
  Hits         2489     2489           
  Misses        234      234           

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[hugovk]

Let's do minor then :)