Drop support for Python 3.7 and 3.8

[mfisher87]

Since dependencies aren't resolvable with 3.7 in conda or pip (and 3.8 unresolvable with conda only), dropping support prior to the final 1.x release

[github-actions]

👈 Launch a binder notebook on this branch for commit 820f163

I will automatically update this comment whenever this PR is modified

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.17%. Comparing base (2566d96) to head (820f163).
Report is 1 commits behind head on development.

❗ There is a different number of reports uploaded between BASE (2566d96) and HEAD (820f163). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (2566d96)	HEAD (820f163)
	3	2

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #608      +/-   ##
===============================================
- Coverage        71.57%   65.17%   -6.41%     
===============================================
  Files               37       37              
  Lines             3061     3061              
  Branches           596      596              
===============================================
- Hits              2191     1995     -196     
- Misses             766      981     +215     
+ Partials           104       85      -19     

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