Fix CI, move to GH

[PGijsbers]

Continuous integration is broken. Instead of fixing it, I want to move to GH CI anyway since it's what I've come to use in other projects.

[PGijsbers]

Partial progress:

• Update versions #158: unit tests
• Add codecov upload to CI #161: codecov

todo:

• update and execute pre-commit with:
  • black format
  • mypy check
• package distribution -> will not do dev releases, should install from repo
• website build & deployment
• test against --pre dependencies'

later:
N system tests (delayed until back-end revision)

[PGijsbers]

Package distribution workflow is on https://github.com/PGijsbers/gama/tree/publish-ci
There are some decisions left:

• automatically publish on push to version branch? or
• automatically publish on tagged release? (requires manual make of github release which is again error prone, but does allow for "releases" on github) => would also require the version file to be updated, tag moved, etc. similar to AMLB.

[PGijsbers]

I consider this done for now, I will integrate the publish action for the 22.0.0 release.

[PGijsbers]

Closing this since #111 already covers the only automation that is not yet completed.