[FIX] switch from np.math to math

[jdkent]

np.math is just a reference to the built in module math (and was removed in recent versions of numpy)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.98%. Comparing base (d0c18bb) to head (176f52f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
+ Coverage   91.09%   91.98%   +0.89%     
==========================================
  Files          13       13              
  Lines         932     1011      +79     
==========================================
+ Hits          849      930      +81     
+ Misses         83       81       -2     

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