Fix: some unit tests failed when using Pandas 2.0

[JohanKJSchreurs]

Fixes issue #176.

Some fixes to make the code compatible with pandas 2.0.

Tests now pass with both pandas v1.5 and v2.0.

For now, I have not yet pinned the pandas version, because we might want to discuss first if we want to leave it unpinned, or what version constraint we should apply. (I suppose we would either pin it to pandas>=2.0 or to pandas<=2.0 )

I don't know if we have users who may be stuck on pandas 1.5.

[soxofaan]

As long as there is no concrete need to add a constraint at the level of openeo-python-driver library, I would leave things as-is.
If we need constraints for the Terrascope/creodias deploys, we can do that in their respective requirements.txt files (e.g. in openeo-deploy project)

[soxofaan]

I've set the pandas constraint to >=1.3.0 for now (1.3.5 works on my machine), to at least have some constraint.

pandas 2 is relatively young at the moment, so maybe better wait it out a bit before deciding to jump ship completely to >=2.0