MNT: Drop Python 3.7 support

[effigies]

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)

Summary

• Drops Python 3.7 support

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.21 ⚠️

Comparison is base (426564e) 81.77% compared to head (c9989ba) 81.56%.

❗ Current head c9989ba differs from pull request most recent head 6ec7755. Consider uploading reports for the commit 6ec7755 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #669      +/-   ##
==========================================
- Coverage   81.77%   81.56%   -0.21%     
==========================================
  Files          20       20              
  Lines        4400     4400              
  Branches     1264        0    -1264     
==========================================
- Hits         3598     3589       -9     
- Misses        798      811      +13     
+ Partials        4        0       -4     

Flag	Coverage Δ
unittests	81.56% <ø> (-0.21%)	⬇️

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see 4 files with indirect coverage changes

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[effigies]

@mgxd Do you know where your Dockerfile is that you used to build https://hub.docker.com/r/mgxd/slurm? We need to update it with a newer Python.

[djarecka]

There is also a newer container that was found here: #667
but I haven't had time to check why some tests are failing