Add mypy to the linting step.

[sodul]

Address mypy issues.
Add CONTRIBUTING.md file.

[sodul]

Tests are failing in CI, I'll reproduce locally and will fix.

[sodul]

For some reason the Ci check for Windows/Py36 got stuck and is still in progress after several hours. I cannot abort it, maybe you can: https://github.com/CleanCut/green/actions/runs/4921522632

To avoid this issue I added a timeout for the CI check. I've added workflow_dispatch so that I could run the checks more easily, on demand, from our fork without having to run it from your side.

[sodul]

The issue with the windows CI check is that I added a tearDown to test_integration.py to cleanup the temp directory. On Windows with Python 3.6 an other process is accessing the same folder somehow, although it is supposed to be a unique folder.
The fix was to ignore errors when calling rmtree, and make it a best effort deletion.

Note that the macOS Py 3.11 check is taking 4-6m to pip install and it seems to be because it needs to compile lxml from source as a universal binary.

@CleanCut let me know if you have any issues with the PR.

[CleanCut]

Making good progress. 😄

[sodul]

@CleanCut There is an ongoing GitHub incident with Actions being impacted. It has been very unstable this week with 3 separate incidents just yesterday.

https://www.githubstatus.com

Feel free to restart the check once the incidents are over, you have more details on this than I do ;-)

[sodul]

@CleanCut sorry to nag you, but is there anything else you need me to update here?

[CleanCut]

Looks good!