fix mixture of sync/async sockets in IOPubThread #2597

Job	Run time
test_without_debugpy (ubuntu-latest, 3.9)	1m 54s
test_docs	40s
build (ubuntu-latest, 3.8)	3m 7s
check_release	1m 35s
build (ubuntu-latest, 3.12)	3m 31s
link_check	3m 18s
build (windows-latest, 3.8)	8m 29s
Make SDist	19s
build (windows-latest, 3.12)	8m 6s
Test Lint	1m 5s
build (macos-latest, 3.8)	5m 42s
Test Prereleases	2m 0s
Test Minimum Versions	2m 22s
build (macos-latest, 3.12)	6m 36s
build (windows-latest, 3.9)	11m 2s
build (ubuntu-latest, pypy-3.9)	3m 3s
build (macos-latest, 3.10)	3m 13s
build (ubuntu-latest, 3.11)	3m 5s
Install from SDist and Test	2m 2s
coverage	11s
tests_check	4s
	1h 11m 24s

Provide feedback