Fix doc build with Sphinx 8. #56

Job	Run time
Check release and publish on twisted-* tag	11s
static-checks	8s
cpython-3.14	26s
cpython-3.11	19s
cpython-3.10	19s
cpython-3.12	22s
cpython-3.13	25s
API docs build	21s
Combine & check coverage.	9s
all-successful	3s
	2m 43s

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