Replace BaseThread's add_task with start_soon #2720

Job	Run time
Make SDist	24s
check_release	1m 47s
build (ubuntu-latest, 3.9)	3m 19s
link_check	3m 34s
build (ubuntu-latest, 3.13)	3m 30s
test_docs	48s
Test Lint	1m 8s
build (windows-latest, 3.9)	8m 55s
Test Minimum Versions	2m 16s
build (windows-latest, 3.13)	10m 17s
Test Prereleases	2m 12s
test_without_debugpy (ubuntu-latest, 3.9)	1m 57s
build (macos-latest, 3.9)	4m 58s
build (macos-latest, 3.13)	2m 54s
build (ubuntu-latest, pypy-3.9)	4m 10s
build (macos-latest, 3.10)	5m 30s
build (ubuntu-latest, 3.11)	2m 56s
build (ubuntu-latest, 3.12)	3m 34s
Install from SDist and Test	2m 17s
coverage	15s
tests_check	2s
	1h 6m 43s

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