Remove downstream_check #2868

Job	Run time
build (ubuntu-latest, 3.9)	2m 53s
Test Lint	43s
build (ubuntu-latest, 3.13)	3m 5s
build (windows-latest, 3.9)	9m 7s
check_release	1m 39s
link_check	3m 27s
build (windows-latest, 3.13)	9m 24s
build (macos-latest, 3.9)	4m 45s
Make SDist	29s
test_docs	39s
build (macos-latest, 3.13)	5m 29s
Test Minimum Versions	2m 9s
build (ubuntu-latest, pypy-3.9)	4m 46s
Test Prereleases	2m 9s
build (macos-latest, 3.10)	5m 9s
test_without_debugpy (ubuntu-latest, 3.9)	2m 3s
build (ubuntu-latest, 3.11)	2m 50s
build (ubuntu-latest, 3.12)	3m 7s
Install from SDist and Test	2m 20s
coverage	7s
tests_check	2s
	1h 6m 22s

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