Maintenance/update to python37

[gro1m]

This PR updates the project to currently supported Python versions, i.e.: Python 3.7, 3.8, 3.9 and 3.10. It also includes the complete migration from setup.py to the static setup.cfg and pyproject.toml to keep-up with current standards. The setup.py shall only contain dynamic content, which was introduced by using pypandoc previously. This package is not needed anymore, as PyPI now supports markdown natively.

[gro1m]

Hi @aconrad So I think in both PRs only the Python3.10 tests do not work and I think it is due to the build configuration, as Ubuntu 16.04 Xenial is deprecated. So we would need minimally Ubuntu 18.04 or Ubuntu 20.04 (LTS versions would of course be better to have, i.e. Bionic Beaver or Focal Fossa). Can you do something there?

[aconrad]

@gro1m can we remove 3.10 support? I feel Travis isn't ready for it, I don't see it listed here:

https://docs.travis-ci.com/user/languages/python/

And people are talking about here, too: https://travis-ci.community/t/add-python-3-10/12220/11

[aconrad]

Oh, and last but not least: can you update the CHANGES that lists the changes done under the "Unreleased" paragraph? And mention yourself, see past contributions as a template. 😉

[gro1m]

Hi @aconrad I hope it looks good now. There is one last thing: You merged the CHANGES.md and the README.md, which I do not do anymore. Doing this also leads to the "problem" that you would use twice the "#"-heading (the title). So I am not quite sure what you want to do there. I do not know yet of an easy option to merge those in the setup.cfg and I am not sure what the correct convention is regarding the title heading. From my viewpoint, it would be enough to have a link to the release notes, but I am not quite sure what you want to do there :)

[aconrad]

There is one last thing: You merged the CHANGES.md and the README.md, which I do not do anymore. [...] I do not know yet of an easy option to merge those in the setup.cfg [...]

Can we do this to concatenate the files?

long_description = file: README.md, CHANGES.md

This seems to be a valid syntax according to: https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

What do you think? Or is there something you said I missed? Not sure I understand the part with headings.

[gro1m]

There is one last thing: You merged the CHANGES.md and the README.md, which I do not do anymore. [...] I do not know yet of an easy option to merge those in the setup.cfg [...]

Can we do this to concatenate the files?

long_description = file: README.md, CHANGES.md

This seems to be a valid syntax according to: https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

What do you think? Or is there something you said I missed? Not sure I understand the part with headings.

That's it - I remembered something, but then I did not find it anymore and I was a bit sceptical when I read that the argument had to be of type string (I did not think of a comma-delimited string).

The thing with the headings is that you should actually not use headings of level 1 ("title") in a markdown document twice, at least a markdown linter will complain, see also: https://github.com/DavidAnson/markdownlint/blob/v0.23.1/doc/Rules.md#md025. But I can live with that.

[aconrad]

Thanks for everything you've done here, i very much appreciate it.