Upgrade Python version to 3.7-3.9

[ConorMacBride]

• Drops support for Python 3.6
• Adds support for Python 3.9
• Min. Numpy 1.17 -> 1.18
• Min. Astropy 3.2 -> 4.2
• Use manylinux2014

Todo

• Change all py39-oldestdeps to py37-oldestdeps
• [ ] Investigate PyYAML import method (try block may not be needed)

[codecov]

Codecov Report

Merging #42 (02d4e1d) into main (e79539c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #42   +/-   ##
=======================================
  Coverage   95.05%   95.05%           
=======================================
  Files          18       18           
  Lines        1376     1376           
=======================================
  Hits         1308     1308           
  Misses         68       68           

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