remove cartopy<0.20 requirement

[gauteh]

No description provided.

[gauteh]

@knutfrode : do you remember why we had this in the first place?

[knutfrode]

No, it seems like you added this 24 days ago, in relation to #708:

ee4db8e#diff-9efd195f4e9bfb79ccd456a1d8370fafcc4bcb0b00ea3799222667d2ae818533

[knutfrode]

But we do get this now:

pyproj.exceptions.ProjError: Error creating Transformer from CRS.: (Internal Proj Error: proj_create_operations: Source and target ellipsoid do not belong to the same celestial body)

[gauteh]

hm yes, seems like tests fail and we need to make some changes in plotting to work with new cartopy.

[knutfrode]

Maybe we can get away with fixes like this:

x, max_y = self.transform_point(lon, lat, PlateCarree())
x, max_y = self.transform_point(lon, lat, PlateCarree(globe=self.globe))

SciTools/cartopy#1252