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 #406 (eb15302) into main (3e8d244) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
+ Coverage   93.84%   93.86%   +0.02%     
==========================================
  Files          65       65              
  Lines       11125    11125              
==========================================
+ Hits        10440    10443       +3     
+ Misses        685      682       -3     

Flag	Coverage Δ
unittests	93.86% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
pyresample/test/test_spherical.py	100.00% <100.00%> (ø)
pyresample/future/resamplers/resampler.py	87.50% <0.00%> (+0.89%)	⬆️
pyresample/utils/rasterio.py	54.38% <0.00%> (+1.75%)	⬆️
pyresample/_compat.py	77.77% <0.00%> (+11.11%)	⬆️

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