Use '.yml' for consistency. #4790

Job	Run time
collateral (diffcov)	5m 16s
collateral (docs)	1m 54s
test (3.8, ubuntu-latest)	4m 6s
collateral (check-extern)	15s
test (3.8, windows-latest)	9m 51s
test_cygwin (39, windows-latest)	14m 32s
test (3.12, ubuntu-latest)	5m 49s
test (3.12, macos-latest)	4m 4s
test (3.12, windows-latest)	11m 55s
test (3.9, ubuntu-latest)	4m 17s
test (3.10, ubuntu-latest)	4m 11s
test (3.11, ubuntu-latest)	3m 41s
test (pypy3.10, ubuntu-latest, stdlib)	6m 51s
test (ubuntu-latest, 3.10, stdlib)	3m 50s
test (3.8, macos-13)	6m 11s
integration-test	0s
check	2s
release	0s
	1h 26m 45s

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