Suggested fix for ImpfSetTropCyclone.get_countries_per_region() #1105

[spjuhel]

Changes proposed in this PR:

• Updates the method to correctly return expected values when providing no argument for region, or None or "all".
• Updates the code for Kosovo in the CountryCode enum to XKO to avoid LookupError.

This PR fixes #1105

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[peanutfun]

Thanks for the fix! Please add a test for region="all".

[peanutfun]

Thanks for adding the test, but it basically doubles the contents of the CountryCode enum, which I think is error-prone. I like the idea of comparing two results of the get_countries_per_region(), though. How about you compare the partial contents of out to get_countries_per_region(reg) for each reg?

out_all = get_countries_per_region("all")
for reg in (...):
    out_reg = get_countries_per_region(reg)
    self.assertDictEqual(out[0][reg], out_all[0])
    # ...and so on