chore: add pyproject.toml to use setuptools features

[mwtoews]

Description

This PR introduces a pyproject.toml file described by PEP 518, and uses PyPA's build to check building the package (sdist and wheel files).

In addition, lots from setup.py is removed or simplified:

• E.g. first few lines are not needed for Python 3, which is UTF-8 by default and has a print function
• Dunders __author__ etc are not normally used in this file
• Remove setuptools check, since this is a build requirement specified in the correct place (pyproject.toml)
• Remove Python version check, since this is done with setuptools' python_requires
• Note that lots of the static metadata from setup.py can be moved to either setup.cfg or pyproject.toml, but not done in this PR

QC

• The PR contains a test case for the changes or the changes are already covered by an existing test case.
• The documentation (docs/) is updated to reflect the changes or this is not necessary (e.g. if the change does neither modify the language nor the behavior or functionalities of Snakemake).

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[johanneskoester]

Thank you!