Remove `BackgroundSocket` wrapper, use `IOPubThread` directly #3974

Job	Run time
Test Lint	52s
test_without_debugpy (ubuntu-latest, 3.10)	7m 12s
Test Minimum Versions	7m 38s
Test Prereleases	7m 30s
Make SDist	30s
test_docs	50s
check_release	1m 54s
link_check	4m 22s
build (ubuntu-latest, qt6, 3.10)	7m 32s
build (windows-latest, qt5, 3.10)	3m 5s
build (macos-latest, qt5, 3.10)	7m 52s
build (windows-latest, qt6, 3.13)	2m 8s
build (ubuntu-latest, qt5, pypy-3.11)	7m 47s
build (ubuntu-latest, qt6, 3.13)	7m 28s
build (windows-latest, qt6, 3.10)	2m 27s
build (windows-latest, qt5, pypy-3.11)	3m 19s
build (windows-latest, qt5, 3.14)	2m 8s
build (macos-latest, qt5, 3.14)	7m 46s
build (macos-latest, qt6, pypy-3.11)	15m 46s
build (macos-latest, qt6, 3.12)	7m 40s
build (macos-latest, qt6, 3.13)	7m 53s
build (ubuntu-latest, qt5, 3.10)	7m 21s
build (ubuntu-latest, qt6, 3.12)	7m 25s
build (ubuntu-latest, qt6, pypy-3.11)	7m 42s
build (windows-latest, qt6, 3.11)	2m 33s
build (windows-latest, qt6, pypy-3.11)	4m 2s
build (macos-latest, qt6, 3.11)	8m 15s
build (ubuntu-latest, qt6, 3.11)	7m 24s
build (macos-latest, qt6, 3.10)	8m 11s
build (macos-latest, qt5, pypy-3.11)	9m 36s
build (ubuntu-latest, qt5, 3.14)	7m 17s
build (windows-latest, qt6, 3.12)	1m 57s
Install from SDist and Test	7m 21s
coverage	0s
	3h 12m 43s

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