Make the test suite work with 3.15

[A5rocks]

Fixes #3355 (hopefully)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.87598%. Comparing base (a97133a) to head (2b7c4cf).
⚠️ Report is 1 commits behind head on main.

❌ Your project check has failed because the head coverage (97.87598%) is below the target coverage (100.00000%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                 main       #3456         +/-   ##
====================================================
- Coverage   100.00000%   97.87598%   -2.12403%     
====================================================
  Files             128         128                 
  Lines           19417       19868        +451     
  Branches         1317        1386         +69     
====================================================
+ Hits            19417       19446         +29     
- Misses              0         415        +415     
- Partials            0           7          +7     

Files with missing lines	Coverage Δ
src/trio/_core/_tests/test_thread_cache.py	99.27007% <100.00000%> (-0.72993%)	⬇️

... and 56 files with indirect coverage changes

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