Set up github actions with tests for each supported python version

[einarf]

• We currently use circleci, but it's far simpler to just use github actions.
• Keeping both is fine. We just need to upgrade the circleci enviroment

Spawned from #124

[einarf]

Tests run with pytest and flake8 on push with github actions using python 3.5, 3.6, 3.7, 3.8 and 3.9