BUG: ensure validity of polygons in voronoi_frames

[martinfleis]

On some of the locations where coverage_union failed, unary union does not fail but produces invalid polygons. If that happens, the subsequent intersection has a tendency to fail. This seems to fix it but I wasn't able to extract a sample small enough to be included in CI. So my proposal is not merge as is.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.3%. Comparing base (4b9970d) to head (5c60509).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
libpysal/cg/voronoi.py	50.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #780     +/-   ##
=======================================
- Coverage   85.4%   85.3%   -0.1%     
=======================================
  Files        150     150             
  Lines      15989   16012     +23     
=======================================
+ Hits       13655   13663      +8     
- Misses      2334    2349     +15     

Files with missing lines	Coverage Δ
libpysal/cg/voronoi.py	75.0% <50.0%> (+1.3%)	⬆️

... and 5 files with indirect coverage changes

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[martinfleis]

Not that easy, the result is wrong...

[martinfleis]

The result is wrong by a coincidence and the wrongness is caused by libgeos/geos#955. This is fine as is.

[jGaboardi]

Nice work!