Updated pycsep requirements for pip installation & ga using pip

[pabloitu]

pip installation was difficult before because cartopy required the system libraries geos and proj. However, from version 0.22.0, cartopy comes with its own wheels to handle those binaries. I updated the reqs by removing geos and proj, but updated the python required version to 3.9.

The github actions for the pycsep-tests and docs were already tested on my fork. Also, added a Dockerfile to create a pycsep image

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Fixes issue #(please fill in or delete if not needed).

Type of change:

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• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes

[fabiolsilva]

Updated requirements and documentation.

[fabiolsilva]

Modified pyCSEP requirements due to updates on cartopy.