Re-enable CI

[FriedrichFroebel]

CI seems to be inactive at the moment. Some stuff has been changed for Travis lately, so it might be a good idea to migrate to GitHub Actions.

I gave it a quick shot today, but ran into some issues which appeared to be related to the GUI. What I have tried:

• Build wxPython from source to build Gamera with GUI support. This resulted in wxPython compilation taking a long time, which I aborted after taking 30 minutes. Building wxPython from source seems to be the only reasonable way to get GUI support working with the ubuntu-latest image (Ubuntu 20.04) and different Python versions (actions/setup-python@v2).
• Build Gamera without GUI support, id est using the --nowx option. When running pytest, there has been a segmentation fault which appeared to be related to importing the gui.py module.

[cdalitz]

What is "CI"?

[FriedrichFroebel]

Continuous Integration, id est automated tests for commits and pull requests. According to https://github.com/hsnr-gamera/gamera-4/blob/master/.travis.yml, Travis CI has been used in the past.

[FriedrichFroebel]

While debugging some issues related with the current release candidate of Python 3.11, I just had a look at this and created a corresponding workflow file which builds and tests the code with the --no-wx option on Ubuntu. After some cleanup, I will probably open a corresponding PR for it. (The wxpython stuff requires us to build the corresponding package manually it I remember correctly, which took about 35 minutes during my last test. A gamera-4 test run takes about 10 minutes, including the source build.)