TST: fix sparse instance assertion

[martinfleis]

Avoiding the private namespace to fix the CI errors with the latest scipy.

[codecov]

Codecov Report

Merging #532 (9404cf4) into main (8fa39a8) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #532     +/-   ##
=======================================
- Coverage   79.9%   79.9%   -0.0%     
=======================================
  Files        113     113             
  Lines      13135   13135             
=======================================
- Hits       10491   10490      -1     
- Misses      2644    2645      +1     

Impacted Files	Coverage Δ
libpysal/weights/tests/test_weights.py	99.7% <100.0%> (ø)

... and 2 files with indirect coverage changes