Drop support for Python 3.6

[kamil-certat]

Python 3.6 is already EOL. We can drop support and update docs, requirements files and package definitions.

Before that, the question should be sent to the mailing list to be sure that no one needs/wants to maintain it.

Follow of #2262

[kamil-certat]

The email was sent.

For clean up:
the harmonization.py contains some compatibility code for Python lower than 3.6

[kamil-certat]

New reason for dropping: Github Action doesn't have Python 3.6 for the Ubuntu 22.04, which now is the ubuntu-latest. This means, our CI is at the moment blocked.

[aaronkaplan]

The email was sent.

For clean up: the harmonization.py contains some compatibility code for Python lower than 3.6

I did not look into that yet.

[aaronkaplan]

keeping open , so far only did the unit test python 3.6 CI removal. See #2281
Still need to look at the code

[sebix]

3.6 is still mentioned in the setup.py