MNT: Diverging software requirements in setup.cfg and requirements*.txt

[adswa]

There are two places that define software requirements in metalad: Two requirements*.txt files and setup.cfg.

I found the following issues:

• The information in setup.cfg and requirements*.txt diverges. E.g., on maint_04 setup.cfg requires datalad-metadata-model >=0.3.10 but requirements.txt (AND requirements-devel.txt) require datalad-metadata-model >=0.3.6

• requirements.txt and requirements-devel.txt have partially overlapping contents. requirements-devel.txt does not only contain devel-only requirements, and requirements.txt likewise contains development requirements (e.g., the entire Sphinx machinery).