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 100.00000%. Comparing base (d0e762f) to head (687af39).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3456   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19439        19450   +11     
  Branches         1318         1321    +3     
===============================================
+ Hits            19439        19450   +11     

Files with missing lines	Coverage Δ
src/trio/_core/_tests/test_thread_cache.py	100.00000% <100.00000%> (ø)
src/trio/_tests/test_exports.py	100.00000% <100.00000%> (ø)
src/trio/socket.py	100.00000% <ø> (ø)

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