Add GitHub Actions

[tfoxy]

Based on the workflows in graphql-python/graphene-django/.github/workflows, but adapted to use the same versions and tools that we were already using in Travis CI so that we can make sure everything works as before. Removed Python 3.6 as it's not supported in the latest version of Ubuntu in GHA.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07% 🎉

Comparison is base (474bc68) 93.63% compared to head (1677204) 93.71%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   93.63%   93.71%   +0.07%     
==========================================
  Files           7        7              
  Lines         330      334       +4     
==========================================
+ Hits          309      313       +4     
  Misses         21       21              

Files Changed	Coverage Δ
graphene_django_optimizer/query.py	92.10% <100.00%> (-0.03%)	⬇️
graphene_django_optimizer/utils.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tfoxy]

@sjdemartini we now have CI working correctly. Didn't check if publishing works. It's almost the same config as graphene-django and I added the secret, so hopefully it works.

I used the same versions and tools of the Travis config to break things as little as possible and to have it working ASAP. But feel free to change anything in the future if it makes things easier/better.

[sjdemartini]

Amazing, thank you @tfoxy! Really appreciate you getting this in place and for releasing the version update. I'll test it out in my project next week 😄