[TEST] Add py3.8 to azure tests

[rmarkello]

Closes #157

[codecov]

Codecov Report

Merging #174 (8d396b9) into main (5b7fbf8) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
+ Coverage   96.30%   96.37%   +0.06%     
==========================================
  Files          37       37              
  Lines        2627     2676      +49     
==========================================
+ Hits         2530     2579      +49     
  Misses         97       97              

Impacted Files	Coverage Δ
abagen/tests/test_io.py	100.00% <0.00%> (ø)
abagen/tests/conftest.py	100.00% <0.00%> (ø)
abagen/tests/test_allen.py	100.00% <0.00%> (ø)
abagen/tests/test_utils.py	100.00% <0.00%> (ø)
abagen/tests/test_images.py	100.00% <0.00%> (ø)
abagen/tests/test_probes.py	100.00% <0.00%> (ø)
abagen/tests/test_samples.py	100.00% <0.00%> (ø)
abagen/tests/test_transforms.py	100.00% <0.00%> (ø)
abagen/tests/mouse/test_mouse.py	100.00% <0.00%> (ø)
abagen/tests/mouse/test_structure.py	100.00% <0.00%> (ø)
... and 3 more

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