Change tested Python versions to 3.8, 3.9 and 3.10

[pnuu]

Drop Python 3.7 from the tested Python versions and add Python 3.10.

[codecov]

Codecov Report

Merging #134 (16c6f61) into main (7bb9c5e) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
+ Coverage   83.14%   83.22%   +0.08%     
==========================================
  Files          22       22              
  Lines        2337     2337              
==========================================
+ Hits         1943     1945       +2     
+ Misses        394      392       -2     

Flag	Coverage Δ
unittests	83.22% <ø> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
pyspectral/tests/test_rsr_reader.py	99.26% <0.00%> (+0.36%)	⬆️
pyspectral/tests/test_reflectance.py	96.62% <0.00%> (+0.67%)	⬆️

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[adybbroe]

LGTM