fix(python): Address indexing edge-case with numpy arrays

[alexander-beedie]

Closes #19882.

• Resolves indexing error/edge-case with np.array(n) on DataFrame and Series __getitem__.
• Adds some extra coverage (and splits out the numpy indexing into its own test).

Also:

• Drive-by fix for an overnight update that triggers some new mypy lint errors (in "test_constructors").

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.43%. Comparing base (f718909) to head (3f45cfb).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/_utils/getitem.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19895      +/-   ##
==========================================
- Coverage   79.44%   79.43%   -0.01%     
==========================================
  Files        1554     1554              
  Lines      215609   215613       +4     
  Branches     2452     2454       +2     
==========================================
- Hits       171281   171279       -2     
- Misses      43770    43775       +5     
- Partials      558      559       +1     

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[ritchie46]

A rebase away.

[alexander-beedie]

A rebase away.

Ahh, @nameexhaustion beat me to the lint drive-by fix; rebasing now :)