Fix type promotion and fix tests for multiple dtypes

[f0uriest]

Resolves #142

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.08%. Comparing base (d2beda9) to head (747f14e).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   98.06%   98.08%   +0.02%     
==========================================
  Files           7        7              
  Lines        1034     1045      +11     
==========================================
+ Hits         1014     1025      +11     
  Misses         20       20              

Files with missing lines	Coverage Δ
interpax/_spline.py	97.80% <100.00%> (+0.04%)	⬆️
interpax/utils.py	93.33% <100.00%> (+0.47%)	⬆️

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