Make kernelbase._eventloop_set event thread-safe #3115

Job	Run time
link_check	1m 43s
check_release	1m 49s
build (ubuntu-latest, 3.9)	2m 1s
build (ubuntu-latest, 3.10)	2m 1s
Make SDist	28s
test_docs	39s
build (ubuntu-latest, 3.11)	2m 1s
build (ubuntu-latest, 3.12)	2m 6s
Test Lint	57s
Test Minimum Versions	1m 59s
build (ubuntu-latest, 3.13)	2m 2s
build (ubuntu-latest, pypy-3.9)	1m 59s
Test Prereleases	1m 57s
build (ubuntu-latest, pypy-3.10)	1m 54s
test_without_debugpy (ubuntu-latest, 3.9)	1m 55s
build (windows-latest, 3.9)	1m 54s
build (windows-latest, 3.10)	1m 53s
build (windows-latest, 3.11)	1m 52s
build (windows-latest, 3.12)	1m 55s
build (windows-latest, 3.13)	1m 53s
build (windows-latest, pypy-3.9)	1m 58s
build (windows-latest, pypy-3.10)	2m 0s
build (macos-latest, 3.9)	1m 59s
build (macos-latest, 3.10)	2m 6s
build (macos-latest, 3.11)	2m 1s
build (macos-latest, 3.12)	2m 2s
build (macos-latest, 3.13)	2m 1s
build (macos-latest, pypy-3.9)	2m 2s
build (macos-latest, pypy-3.10)	2m 0s
Install from SDist and Test	1m 21s
coverage	1s
	54m 29s

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