REF: rely on libpysal for kernels

[martinfleis]

This needs libpysal main + two PRs that are yet to be merged. Keeping as a draft until then. We'll also need to bump libpysal min version once that is released.

Update: This PR removes custom handling of kernels and relies on new functionality from libpysal 4.15. Pinning the minimum to that and removing all the custom code from spml.

[martinfleis]

This is now green on dev and waits only for lib release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.34%. Comparing base (0b4f954) to head (ccc0f8a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
- Coverage   92.39%   92.34%   -0.06%     
==========================================
  Files           6        6              
  Lines         881      849      -32     
==========================================
- Hits          814      784      -30     
+ Misses         67       65       -2     

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