ci: add Slack notification for maintainer PRs #937

Job	Run time
checks / pre-commit	43s
checks / readme-snippets	13s
checks / test (3.10, locked, windows-latest)	2m 17s
checks / test (3.11, lowest-direct, windows-latest)	2m 35s
checks / test (3.12, lowest-direct, windows-latest)	3m 43s
checks / test (3.12, locked, windows-latest)	3m 18s
checks / test (3.13, locked, ubuntu-latest)	46s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 5s
checks / test (3.14, locked, windows-latest)	3m 32s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.13, locked, windows-latest)	2m 19s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.11, lowest-direct, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, windows-latest)	2m 25s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.14, lowest-direct, windows-latest)	2m 23s
checks / test (3.11, locked, windows-latest)	3m 0s
checks / test (3.10, lowest-direct, windows-latest)	2m 56s
checks / test (3.14, locked, ubuntu-latest)	2m 23s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
all-green	8s
Notify maintainers	2s
	40m 24s

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