Simplify vendoring and declare dependencies optionally #4850

Job	Run time
test_cygwin (39, windows-latest)	15m 16s
test (3.8, ubuntu-latest)	3m 40s
collateral (diffcov)	5m 16s
test (3.8, windows-latest)	11m 25s
collateral (docs)	1m 52s
test (3.12, ubuntu-latest)	5m 30s
test (3.12, macos-latest)	4m 7s
test (3.12, windows-latest)	11m 22s
test (3.9, ubuntu-latest)	3m 59s
test (3.10, ubuntu-latest)	3m 42s
test (3.11, ubuntu-latest)	3m 54s
test (pypy3.10, ubuntu-latest, stdlib)	11m 42s
test (ubuntu-latest, 3.10, stdlib)	3m 53s
test (3.8, macos-13)	6m 20s
integration-test	0s
check	2s
release	0s
	1h 32m 0s

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