Update sphinx-issues requirement from <5 to <6 #1340

Job	Run time
Run tests on ubuntu-latest, py3.8	1m 34s
Run tests on ubuntu-latest, py3.9	1m 29s
Run tests on ubuntu-latest, py3.10	1m 34s
Run tests on ubuntu-latest, py3.11	1m 28s
Run tests on ubuntu-latest, py3.12	1m 46s
Run tests on windows-latest, py3.8	2m 32s
Run tests on windows-latest, py3.9	2m 21s
Run tests on windows-latest, py3.10	2m 33s
Run tests on windows-latest, py3.11	3m 2s
Run 'mypy' on python 3.x	23s
Run tests on windows-latest, py3.12	2m 53s
Run 'twine-check' on python 3.x	17s
test-sdist	1m 36s
Run tests on macos-latest, py3.8	1m 28s
Run tests on macos-latest, py3.9	1m 13s
Run tests on macos-latest, py3.10	1m 13s
Run 'py-mindeps' on python 3.8	29s
Run tests on macos-latest, py3.11	1m 5s
ensure-catalog-matches-hook-config	19s
Run tests on macos-latest, py3.12	1m 9s
Self-Check	12s
Collate results to check for skipped tests	8s
	30m 44s

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