Subshells implemented using tornado event loops on 6.x branch #3320

Job	Run time
build (ubuntu-latest, 3.9)	9m 49s
check_release	1m 52s
link_check	1m 48s
Make SDist	36s
build (ubuntu-latest, 3.13)	7m 46s
build (windows-latest, 3.9)	8m 2s
test_docs	38s
Test Lint	1m 12s
Test Minimum Versions	2m 24s
build (windows-latest, 3.13)	7m 15s
Test Prereleases	2m 14s
build (macos-latest, 3.9)	5m 2s
test_without_debugpy (ubuntu-latest, 3.9)	2m 11s
build (macos-latest, 3.13)	10m 2s
build (ubuntu-latest, pypy-3.9)	4m 46s
build (macos-latest, 3.10)	4m 42s
build (ubuntu-latest, 3.11)	4m 52s
build (ubuntu-latest, 3.12)	8m 8s
Install from SDist and Test	2m 26s
coverage	0s
	1h 25m 45s

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