Drop python 3.7

[nstarman]

Signed-off-by: Nathaniel Starkman (@nstarman) nstarkman@protonmail.com

To match astropy.

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• Document changes in the CHANGES.rst file. See existing changelog for examples.
• Add tests, if applicable, to ensure code coverage never decreases.
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[codecov]

Codecov Report

Merging #64 (7665361) into main (3b17b6e) will decrease coverage by 16.04%.
The diff coverage is 58.06%.

@@             Coverage Diff             @@
##             main      #64       +/-   ##
===========================================
- Coverage   88.28%   72.23%   -16.05%     
===========================================
  Files          33       33               
  Lines        1673     1675        +2     
===========================================
- Hits         1477     1210      -267     
- Misses        196      465      +269     

Impacted Files	Coverage Δ
discO/core/__init__.py	100.00% <ø> (ø)
discO/core/common.py	100.00% <ø> (ø)
discO/core/pipeline.py	87.78% <ø> (ø)
discO/plugin/__init__.py	55.55% <ø> (-22.23%)	⬇️
discO/plugin/agama/__init__.py	0.00% <ø> (ø)
discO/plugin/agama/fitter.py	0.00% <0.00%> (ø)
discO/plugin/agama/sample.py	0.00% <0.00%> (ø)
discO/plugin/agama/type_hints.py	0.00% <ø> (ø)
discO/plugin/agama/wrapper.py	0.00% <ø> (ø)
discO/plugin/gala/__init__.py	0.00% <ø> (-100.00%)	⬇️
... and 35 more

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