ci: add Slack notification for maintainer PRs #935

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	40s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.11, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.10, lowest-direct, windows-latest)	2m 18s
checks / test (3.11, locked, ubuntu-latest)	51s
checks / test (3.11, lowest-direct, windows-latest)	2m 44s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.11, locked, windows-latest)	2m 15s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 26s
checks / test (3.12, locked, windows-latest)	3m 26s
checks / test (3.14, lowest-direct, windows-latest)	2m 13s
checks / test (3.13, locked, windows-latest)	2m 37s
checks / test (3.12, lowest-direct, windows-latest)	3m 30s
checks / test (3.13, lowest-direct, windows-latest)	2m 45s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.10, locked, windows-latest)	2m 2s
checks / test (3.14, locked, windows-latest)	3m 5s
checks / test (3.14, locked, ubuntu-latest)	2m 23s
all-green	5s
Notify maintainers	3s
	39m 4s

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