Log a warning if the Rust extension doesn't import

[puddly]

See NabuCasa/universal-silabs-flasher#143.

Free-threaded Python on macOS doesn't support abi3 and thus the macOS wheel can't be pip installed. Instead, the pure Python wheel gets installed. This fails to load because we assume the Rust extension is present within the macOS module. We can't support free-threaded Python wheels at the moment due to there being no stable ABI (maybe once Python 3.15 is released). The next best thing is to degrade gracefully.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.77%. Comparing base (e062529) to head (6d34d24).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
serialx/serialx_rust.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #74   +/-   ##
=======================================
  Coverage   90.77%   90.77%           
=======================================
  Files          21       22    +1     
  Lines        3349     3360   +11     
=======================================
+ Hits         3040     3050   +10     
- Misses        309      310    +1     

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