feat: Add GitHub actions for pull requests/master build

[markstory]

The release build jobs will be moved once these are working. Also import a basic mypy configuration file.

[codecov]

Codecov Report

Merging #366 (634a225) into master (a7d15eb) will increase coverage by 0.16%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
+ Coverage   96.25%   96.42%   +0.16%     
==========================================
  Files           3        3              
  Lines        1282     1286       +4     
==========================================
+ Hits         1234     1240       +6     
+ Misses         48       46       -2     

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
responses/test_responses.py	98.07% <83.33%> (+0.25%)	⬆️

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