fix(python): Fix return type of Series.dt.add_business_days

[Terrigible]

The return type of Series.dt.add_business_days should be Series, not Expr

>>> import polars as pl
>>> import datetime

>>> type(
    pl.Series("date", [datetime.date(2024, 12, 23)]).dt.add_business_days(
        1, roll="forward"
    )
)
<class 'polars.series.series.Series'>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.96%. Comparing base (127f590) to head (6b0b120).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20402      +/-   ##
==========================================
- Coverage   78.96%   78.96%   -0.01%     
==========================================
  Files        1562     1562              
  Lines      220078   220078              
  Branches     2480     2480              
==========================================
- Hits       173793   173789       -4     
- Misses      45714    45718       +4     
  Partials      571      571              

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[MarcoGorelli]

Thanks @Terrigible !