COMPAT: make greedy compatible with future

[martinfleis]

Closes #209

I have no clue which package causes this, probably some weird interaction between multiple nightly wheels. No clue, I really tried to debug it. Gave up and opted for an explicit check that the result is indeed greedy instead. We don't care about the counts, we only care if the colouring ensures two adjacent polygons do not share the colour.

Also updated use_index usage to silence the warnings.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.4%. Comparing base (17ae858) to head (28b851e).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #210   +/-   ##
=====================================
  Coverage   89.4%   89.4%           
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  Files          8       8           
  Lines       1088    1088           
=====================================
  Hits         973     973           
  Misses       115     115           

Files	Coverage Δ
mapclassify/greedy.py	100.0% <100.0%> (ø)