chore(python): Initial PyO3 0.21 support

[itamarst]

Fixes #15621

Next step will probably be getting rid of the manual deprecation suppression by switching to the non-deprecated numpy APIs, in the hopes that's somewhat self-contained.

[codspeed-hq]

CodSpeed Performance Report

Merging #15622 will degrade performances by 10.98%

_{Comparing itamarst:15621-initial-pyo3-0.21-support (3d78fb2) with main (92902e6)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	itamarst:15621-initial-pyo3-0.21-support	Change
❌	test_sort_nan_1942	1 ms	1.2 ms	-10.98%

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.26%. Comparing base (92902e6) to head (3d78fb2).

Files	Patch %	Lines
py-polars/src/conversion/mod.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15622   +/-   ##
=======================================
  Coverage   81.25%   81.26%           
=======================================
  Files        1371     1371           
  Lines      175772   175778    +6     
  Branches     2531     2531           
=======================================
+ Hits       142831   142839    +8     
+ Misses      32464    32463    -1     
+ Partials      477      476    -1     

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