Relax test_leak to run reliable against free-threaded python

[asvetlov]

Free-threaded Python has a little different GC implementation, the number in the test should be raised.

[codspeed-hq]

CodSpeed Performance Report

Merging #1060 will degrade performances by 9.8%

_{Comparing relax (40add31) with master (fe4dee0)}

Summary

❌ 1 (👁 1) regressions
✅ 37 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
👁	test_multidict_extend_str	168.8 µs	187.1 µs	-9.8%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.53%. Comparing base (fe4dee0) to head (40add31).
Report is 1 commits behind head on master.

❌ Your project status has failed because the head coverage (86.53%) is below the target coverage (100.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1060   +/-   ##
=======================================
  Coverage   97.53%   97.53%           
=======================================
  Files          22       22           
  Lines        2597     2597           
  Branches      376      376           
=======================================
  Hits         2533     2533           
  Misses         36       36           
  Partials       28       28           

Flag	Coverage Δ
CI-GHA	97.53% <100.00%> (ø)
MyPy	86.53% <100.00%> (ø)
pytest	98.18% <100.00%> (ø)

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