Drop support for Python 3.6 and 3.7

[JulioAPeraza]

No description provided.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.79%. Comparing base (604bb4f) to head (3cfb86b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
- Coverage   88.01%   87.79%   -0.23%     
==========================================
  Files          13       13              
  Lines         893      893              
==========================================
- Hits          786      784       -2     
- Misses        107      109       +2     

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[JulioAPeraza]

@jdkent , I had to open a separate PR to drop support for python 3.6, and 3.7. Please, have a look at it when you get the time.

After merging this one, #117 should be ready for review.

Thank you!

[jdkent]

LGTM!

Took a look at our dependents, looks like we aren't too depended on for anything pre python 3.8:

https://github.com/neurostuff/PyMARE/network/dependents?dependent_type=REPOSITORY