perf(python): Further optimise initial polars import

[alexander-beedie]

Shaves another few ms off the initial polars import; definitely don't need Excel utility functions in the top-level DataFrame imports, and can avoid a premature io utils import too.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.77%. Comparing base (6d48c11) to head (3c238f0).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16308      +/-   ##
==========================================
- Coverage   80.78%   80.77%   -0.01%     
==========================================
  Files        1393     1393              
  Lines      179362   179416      +54     
  Branches     2921     2921              
==========================================
+ Hits       144894   144926      +32     
- Misses      33965    33987      +22     
  Partials      503      503              

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