Cleanup CIs

[skjerns]

We currently have several CIs runnig with several different trigger points.

I think, running everything through GitHub Actions should be cleanest and work well.

For now I propose:

• Remove CircleCI
  • seems to give build error anyway
  • @holgern would need to disable it in on their website
• Remove Appveyor
  • takes forever to build
  • I see no advantage over Actions

Only one thing I'm not sure about: Both Actions and AppVeyor should use VS2019 to build on Windows. I'm not sure if they differ here and we get a greater variance of build chains that we cover.

Additionally:

• Is tox actually used? Or can we also remove it? @holgern
• I'd like to include a workflow that automatically triggers on making a release, creates wheels for all platforms and uploads them to PiPy, what do you think?

[holgern]

I disabled CircleCI and agree that we do not need that.

[holgern]

Visual Studio 2019 is used by appveyer and github actions. Appveyer is using some older VS for python 3.6-3.8

we can change the github action script and add windows-2016 besides windows-2019 in order to guarantee that VS2019 und VS2017 are used. I think this is sufficient for testing. https://github.com/actions/virtual-environments/blob/main/images/win/Windows2016-Readme.md https://github.com/actions/virtual-environments/blob/main/images/win/Windows2019-Readme.md

I think we can disable also AppVeyoer.

tox was used by travis, so I think it can also be removed.