MNT: Drop Python 3.7 support, test through 3.11

[effigies]

Some additional cleanups. Not touching CircleCI right now, as it doesn't seem broken and it's at least testing 3.9.

Also ran pyupgrade --py38-plus. Automated housekeeping seems like a win.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -1.65 ⚠️

Comparison is base (2009ceb) 98.59% compared to head (b1ca471) 96.95%.

❗ Current head b1ca471 differs from pull request most recent head 300ae9d. Consider uploading reports for the commit 300ae9d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   98.59%   96.95%   -1.65%     
==========================================
  Files          13       13              
  Lines        1279     1279              
  Branches      184      184              
==========================================
- Hits         1261     1240      -21     
- Misses         10       33      +23     
+ Partials        8        6       -2     

Flag	Coverage Δ
travis	?
unittests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

see 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[effigies]

This should be uncontroversial. OTOH, dropping a Python version should probably result in a minor release, so I don't know if you want to wait until you're ready to make 23.0.x releases require a maintenance branch.

[mgxd]

As this package was created as a stopgap prior to nibabel integration, I'm not convinced there should be any maintenance branches needed - we can always just cut 23.1.0 if needed