chore(python): Update plugin example to PyO3 0.21

[itamarst]

Fixes #16155

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.94%. Comparing base (b38aa4b) to head (6bda164).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16157      +/-   ##
==========================================
- Coverage   80.95%   80.94%   -0.01%     
==========================================
  Files        1386     1386              
  Lines      178423   178423              
  Branches     2881     2881              
==========================================
- Hits       144437   144433       -4     
- Misses      33493    33498       +5     
+ Partials      493      492       -1     

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