build(python): Fix allocator features

[stinodego]

@ritchie46 I expect this to resolve the issue, but I don't really have a good way of testing it except for trying the release again.

I also changed the build features from use_mimalloc and default_allocator to a feature with two possible values allocator = "default" or allocator = "mimalloc". That is not required for the fix but should be a bit more sensible.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.36%. Comparing base (6289dc3) to head (250c595).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16365      +/-   ##
==========================================
- Coverage   81.38%   81.36%   -0.02%     
==========================================
  Files        1406     1406              
  Lines      183911   183911              
  Branches     2953     2953              
==========================================
- Hits       149671   149647      -24     
- Misses      33727    33751      +24     
  Partials      513      513              

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