tests: add no pandas dependency test of vals function

[machow]

This PR adds a test of a vals function, that demonstrates it working on a Polars series, without having Pandas installed.

This is intended to provide a simple test of #675, showing Pandas is not required to use vals. However, it's fairly simple, so more testing / looking around may be needed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.18%. Comparing base (4a11db5) to head (9dafff7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #689   +/-   ##
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  Coverage   91.18%   91.18%           
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  Files          47       47           
  Lines        5465     5465           
=======================================
  Hits         4983     4983           
  Misses        482      482           

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[rich-iannone]

LGTM!