Add Mergify schema + hook #1313

Job	Run time
Self-Check	18s
Run tests on ubuntu-latest, py3.8	1m 0s
ensure-catalog-matches-hook-config	18s
Run 'mypy' on python 3.x	27s
test-sdist	1m 35s
Run tests on ubuntu-latest, py3.9	57s
Run 'twine-check' on python 3.x	22s
Run tests on ubuntu-latest, py3.10	1m 0s
Run tests on ubuntu-latest, py3.11	54s
Run 'py-mindeps' on python 3.8	29s
Run tests on ubuntu-latest, py3.12	1m 22s
Run tests on windows-latest, py3.8	1m 53s
Run tests on windows-latest, py3.9	1m 39s
Run tests on windows-latest, py3.10	2m 28s
Run tests on windows-latest, py3.11	2m 2s
Run tests on windows-latest, py3.12	2m 3s
Run tests on macos-latest, py3.8	1m 1s
Run tests on macos-latest, py3.9	47s
Run tests on macos-latest, py3.10	52s
Run tests on macos-latest, py3.11	42s
Run tests on macos-latest, py3.12	1m 7s
Collate results to check for skipped tests	4s
	23m 20s

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