dependency version management

[rustamdantia]

• Create several setup.cfg files as in Augusto's comment below
• setup KML/fiona imports dynamically to work with old and new versions of geopandas new old
• do we need to add instructions for GDAL as an external dependency to be manually installed?
• Decide which python version range we are going to support. (from __future__ import annotations probably requires 3.7)

Also, now that I think about it, it might be premature to delete the requirements.txt file. The problem is that a library should be compatible with dependency version ranges as broad as possible; on the other hand, we want to have a stable development environment so that if something breaks, we know it's on our side and not some dependency issue.

I'm afraid there's no easy way to handle this, but it would go along these lines: create not just one requirements.txt but rather a series of them, with various dependency versions (old, newish, latest), and then run the unit tests on the CI for each variation.

Originally posted by @astoff in #30 (comment)