Use setup.cfg to define most package metadata

[mkniewallner]

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Define package metadata and requirements in setup.cfg, with regards to setuptools documentation, allowing to have declarative configuration which should be easier to maintain.

Tested in mkniewallner#5, with the source published here.
Taking the occasion to add python_requires and project_urls to the metadata, in order to have link to issues, source code, changelog and documentation directly from PyPI.
tests_require is removed because it's been deprecated in favor of extras_require (pypa/setuptools#1684)

Note that dependabot will continue to work as expected, as support for setup.cfg has been added a few months ago.