fix(python): Fix decimal series dispatch

[ritchie46]

fixes #20369

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.95%. Comparing base (234810d) to head (af40df2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/series/series.py	81.81%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20400      +/-   ##
==========================================
- Coverage   78.95%   78.95%   -0.01%     
==========================================
  Files        1561     1561              
  Lines      219913   219931      +18     
  Branches     2471     2480       +9     
==========================================
+ Hits       173643   173647       +4     
- Misses      45702    45713      +11     
- Partials      568      571       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.