Remove np.warnings from fairness_utils.py.

[desilinguist]

Closes #579.

Numpy v1.24 removes the warnings alias so we need to use warnings directly in fairness_utils.py.

[codecov]

Codecov Report

Base: 93.08% // Head: 93.08% // No change to project coverage 👍

Coverage data is based on head (f12ed79) compared to base (272e97e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #580   +/-   ##
=======================================
  Coverage   93.08%   93.08%           
=======================================
  Files          31       31           
  Lines        4529     4529           
=======================================
  Hits         4216     4216           
  Misses        313      313           

Impacted Files	Coverage Δ
rsmtool/fairness_utils.py	86.95% <100.00%> (ø)

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