Replace BaseThread's add_task with start_soon #2926

Job	Run time
build (ubuntu-latest, 3.9)	3m 0s
check_release	1m 43s
link_check	3m 11s
build (ubuntu-latest, 3.13)	2m 53s
Make SDist	20s
build (windows-latest, 3.9)	7m 47s
test_docs	43s
build (windows-latest, 3.13)	16m 22s
Test Lint	57s
Test Minimum Versions	2m 14s
build (macos-latest, 3.9)	5m 53s
Test Prereleases	1m 55s
build (macos-latest, 3.13)	5m 3s
test_without_debugpy (ubuntu-latest, 3.9)	1m 51s
build (ubuntu-latest, pypy-3.9)	4m 10s
build (macos-latest, 3.10)	2m 58s
build (ubuntu-latest, 3.11)	2m 45s
build (ubuntu-latest, 3.12)	3m 2s
Install from SDist and Test	1m 59s
coverage	12s
tests_check	2s
	1h 9m 0s

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