[ENH] address readthedocs build failures

[fkiraly]

This PR solves the readthedocs build issue by:

• isolating some numpy imports
• adding the test dep set as required for readthedocs builds to avoid import failures (pytest)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e2271b5) 84.18% compared to head (9175e4a) 84.19%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
+ Coverage   84.18%   84.19%   +0.01%     
==========================================
  Files          43       43              
  Lines        2889     2891       +2     
==========================================
+ Hits         2432     2434       +2     
  Misses        457      457              

Files	Coverage Δ
skbase/utils/tests/test_check.py	100.00% <100.00%> (ø)
skbase/utils/tests/test_deep_equals.py	100.00% <100.00%> (ø)

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