chore(python): Clean up some remnants of Python 3.8 support

[stinodego]

There was still a bunch of logic specific to Python 3.8 in the code base.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.61%. Comparing base (84d317a) to head (b9de395).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/_utils/convert.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20293   +/-   ##
=======================================
  Coverage   79.61%   79.61%           
=======================================
  Files        1565     1565           
  Lines      218328   218319    -9     
  Branches     2478     2477    -1     
=======================================
- Hits       173820   173817    -3     
+ Misses      43941    43936    -5     
+ Partials      567      566    -1     

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[alexander-beedie]

Nice! Removing dead code counts for double points... :)