chore: make Duration.is_constant_duration less strict for non-timezone-aware case

[MarcoGorelli]

splitting this off from #15638

There's currently a method Duration.is_constant_duration, which returns false if the duration contains calendar durations (months, weeks, days).
However, for the time-zone-naive case (and the UTC case), days and weeks are constant durations too, as they're not affected by daylight saving time

There's no implication for the current tests (but it will help with #15638, as there half_life has to be a constant duration)

[codspeed-hq]

CodSpeed Performance Report

Merging #15639 will not alter performance

_{Comparing MarcoGorelli:simplify-is-constant-duration (019ff08) with main (4eef5b5)}

Summary

✅ 22 untouched benchmarks

[codecov]

Codecov Report

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

Project coverage is 81.30%. Comparing base (7341aee) to head (019ff08).

Files	Patch %	Lines
crates/polars-time/src/windows/duration.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15639   +/-   ##
=======================================
  Coverage   81.30%   81.30%           
=======================================
  Files        1373     1373           
  Lines      176243   176245    +2     
  Branches     2544     2544           
=======================================
+ Hits       143293   143299    +6     
+ Misses      32466    32463    -3     
+ Partials      484      483    -1     

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[ritchie46]

Don't see us going in the less strict one often. ^^