allow y from pandas

[mlondschien]

• to_numpy in _check_test_inputs
• Move tests.utils -> utils.
• Rename _check_test_inputs -> _check_inputs
• Use check_inputs in KClass and Summary.
• Changelog.

[codecov]

Codecov Report

Attention: Patch coverage is 88.98305% with 13 lines in your changes missing coverage. Please review.

Project coverage is 94.96%. Comparing base (489914b) to head (f2fda25).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
ivmodels/utils.py	82.85%	12 Missing ⚠️
ivmodels/summary.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   94.79%   94.96%   +0.16%     
==========================================
  Files          36       35       -1     
  Lines        2153     2163      +10     
==========================================
+ Hits         2041     2054      +13     
+ Misses        112      109       -3     

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