Tests/add GitHub actions

[V0lantis]

This PR repaires failing tests and add Github Actions jobs, to tests every PR to enforce some sort of codes quality.

I have tested this in my own fork, if you want to see the result here

[mocnik-science]

Thanks for the code. I will check this in detail and might adapt it in parts. This will most likely happen in October or November.

[V0lantis]

No worries :)

[mocnik-science]

I found some unexpected time even before October, so I have checked the changes you have proposed already now.

• Regarding the proposed change of the use case in the documentation related to the testing, please see your pull request #59.

• The change of .gitignore: I do not see why this should be changed.

• Regarding the GitHub Action “GitHub Actions Demo” that you have introduced: while testing is a nice and useful idea, I have experienced issues with testing on a server. Local testing is much easier for me, because I have more control over error messages popping up related to issues with Overpass server load. Also, introducing such additional complexity needs a lot of tweaking and testing. Here, one should not forget that I am the only one introducing changes, which is why I can easily test before submitting.

• Regarding the ISO3166-2-lvl4: Thanks, there was a need to insert this attribute because Nominatim changed their response. Appreciated and incorporated into the repository.

• Regarding your comment If n get too large, we might experience a _504 Gateway Timeout_ error: The intention is to test here whether OSMPythonTools is able to deal with the the Overpass server receiving too many queries at the same time. In this case, the Overpass server answers that OSMPythonTools needs to wait for some time, and this is exactly what happens. Lowering n (like you proposed) would basically render the test useless. If you receive a 504 error, this means that the Overpass server was down for a while. I have tested again, the test works like intended.