Migrate to github actions from travis CI

[XiangRongLin]

• https://github.com/actions/setup-python

currently testing it: XiangRongLin#1

[XiangRongLin]

This can only be done after #33

[TheAssassin]

I could imagine it's much easier to just run your commands in a python:3 Docker image. Mind if I try that out?

[XiangRongLin]

Feel free to take over. I added some stuff that o could fix quickly (and dirty).

---

I managed to run it locally after i figured out to install the dependencies in user mode, meaning using pip install --user.

Before that i was using nectos/act but that took way to long

[TheAssassin]

I'll add a shell script that is going to allow you to run it locally as the CI would. This is a pattern I use in most repositories. Being able to reproduce results locally is important.

[XiangRongLin]

That what i use https://github.com/nektos/act for. But it's big and compativly slow and with many caveats

[TheAssassin]

I guess we have to merge this initially in order to see if it works. In my temporary fork, it looks good, though.

For the record: this is one of the reasons I love sphinx-doc. It provides this feature out of the box.

[XiangRongLin]

@TheAssassin I collected the problems i have found in #33. Of the 2 remaining problems, only 4 is critical in order to have the CI pipeline run successfully.
The script probably needs to adjusted so that it either

1. reads in all files at the start (con: growing memory load, should be ok with the current 64kB)
2. or load the referenced file when needed (con: same file gets loaded multiple times)
   so one can check references to other files.

I would be in favor of 1

[TheAssassin]

Either will work. There is really no performance optimization required. I mean, if you really wanted, you could implement a lazy loader that caches the contents...