Feature/gha tests dependency caching

[CrepeGoat]

references:

• https://medium.com/ai2-blog/python-caching-in-github-actions-e9452698e98d
• the cache action
• the cache action - pip examples

[codecov-commenter]

Codecov Report

Merging #102 (3453309) into develop (e0e7844) will increase coverage by 5.15%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #102      +/-   ##
===========================================
+ Coverage    69.05%   74.21%   +5.15%     
===========================================
  Files           10       10              
  Lines         3639     3641       +2     
===========================================
+ Hits          2513     2702     +189     
+ Misses        1126      939     -187     

Impacted Files	Coverage Δ
idelib/matfile.py	71.27% <0.00%> (+50.15%)	⬆️

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[CrepeGoat]

most recent tests show that dependencies are being pulled from the cache (see e.g., here) -> appears to be working now

[CFlaniganMide]

while I appreciate the changes, I think that caching dependencies on private repos adds unnecessary moving parts.

[CFlaniganMide]

It looks like the hash isn't being calculated correctly, the action is reporting the key as:

Cache restored successfully
Cache restored from key: Linux-pip-