MAINT: move static metadata to setup.cfg

[mwtoews]

This PR is a follow-up from #2419, and moves the project metatdata from setup.py to a declarative configuration setup.cfg file.

• New metadata are license_files and classifiers
• Option packages is changed from explicit to find:, which adds geopandas.io.tests. Was this intentionally excluded, or was it missed?
• Option package_data is changed from custom-generated to an equivalent list of datasets
• Option install_requires is moved to setup.cfg. It was not clear why this would be empty for ReadTheDocs, which has these dependencies met with doc/environment.yml. However, if there is a good reason, this dynamic option can be moved back to setup.py.
• Most of setup.py is there to support versioneer or anyone that expects to see this file.

[jorisvandenbossche]

• Option packages is changed from explicit to find:, which adds geopandas.io.tests. Was this intentionally excluded, or was it missed?

Missed by mistake I suppose

• Option install_requires is moved to setup.cfg. It was not clear why this would be empty for ReadTheDocs, which has these dependencies met with doc/environment.yml. However, if there is a good reason, this dynamic option can be moved back to setup.py.

I think the reason for this was to avoid pip upgrading some of the dependencies from the conda environment (Readthedocs installs the checkout using pip install --upgrade .). We would need to check on RTD if this still gives problems nowadays, but we don't have RTD builds enabled on PRs ..

[jorisvandenbossche]

I think the reason for this was to avoid pip upgrading some of the dependencies from the conda environment

Although this bit of code stems from a time before we used conda (#255), so it might not actually be needed anymore. I can check that on a branch in the main repo that gets temporarily enabled on RTD.

[mwtoews]

See #2512