[misc] Migrate CI to GitHub Actions

[slarse]

Fix #15

[codecov]

Codecov Report

Merging #17 (0e12740) into master (9f0e356) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   96.15%   96.10%   -0.05%     
==========================================
  Files           3        3              
  Lines          78       77       -1     
  Branches       14       14              
==========================================
- Hits           75       74       -1     
  Misses          2        2              
  Partials        1        1              

Impacted Files	Coverage Δ
repobee_feedback/feedback.py	96.00% <ø> (-0.06%)	⬇️

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