DEV: remove geopandas from the environment.yml file

[jorisvandenbossche]

GeoPandas is a heavy dependency (though its own dependencies, including GDAL), and is only used for a single test (in test_downstream.py). So it might be a better option to remove geopandas from the environment.yml file, to ensure that not every contributor needs to install it when creating a development environment.

[StefanieSenger]

take