BUG: fix weights plotting

[martinfleis]

Closes #525

We currently do not have any test suite for plotting, so I am not adding any tests here. Will add some as part of the weights refactoring which will include plotting refactoring as well.

[jGaboardi]

Failure:

FAILED libpysal/weights/tests/test_weights.py::TestW::test_to_sparse - AttributeError: module 'scipy.sparse' has no attribute '_arrays'

[martinfleis]

@jGaboardi that is the one I fixed in parallel in #532

[codecov]

Codecov Report

Merging #533 (e44debc) into main (d925cc3) will increase coverage by 0.1%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main    #533     +/-   ##
=======================================
+ Coverage   79.9%   79.9%   +0.1%     
=======================================
  Files        113     113             
  Lines      13135   13135             
=======================================
+ Hits       10491   10500      +9     
+ Misses      2644    2635      -9     

Impacted Files	Coverage Δ
libpysal/weights/weights.py	83.2% <0.0%> (ø)

... and 1 file with indirect coverage changes

[jGaboardi]

re: plotting tests

I've had success with matplotlib's decorator – image_comparison

We should probably open a specific issue for this.

[martinfleis]

I think we have some issue like this in splot?

[jGaboardi]

xref pysal/splot#145