fix(python): ChainedWhen should not inherit Expr

[eitsupi]

Perhaps it was a miss copy that was confused with Then or ChainedThen.

>>> import polars as pl
>>> pl.when(True).then(1).when(True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vscode/.local/lib/python3.9/site-packages/polars/expr/expr.py", line 137, in __repr__
    if len(expr_str := self._pyexpr.to_str()) > 30:
AttributeError: 'NoneType' object has no attribute 'to_str'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.87%. Comparing base (3444f64) to head (c157365).
Report is 4 commits behind head on main.

❗ Current head c157365 differs from pull request most recent head 2c318e3

Please upload reports for the commit 2c318e3 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17142      +/-   ##
==========================================
- Coverage   80.88%   80.87%   -0.02%     
==========================================
  Files        1456     1456              
  Lines      191333   191333              
  Branches     2742     2742              
==========================================
- Hits       154757   154732      -25     
- Misses      36067    36092      +25     
  Partials      509      509              

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[stinodego]

Thanks, you're right! My mistake.

Thanks for noticing and opening a PR.