ci: add Slack notification for maintainer PRs #939

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	13s
checks / test (3.10, lowest-direct, windows-latest)	2m 29s
checks / test (3.11, lowest-direct, ubuntu-latest)	57s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.11, locked, windows-latest)	3m 32s
checks / test (3.10, locked, windows-latest)	2m 25s
checks / test (3.12, locked, windows-latest)	3m 26s
checks / test (3.14, locked, ubuntu-latest)	2m 24s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.11, lowest-direct, windows-latest)	3m 9s
checks / test (3.13, lowest-direct, windows-latest)	2m 43s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.14, locked, windows-latest)	3m 39s
checks / test (3.13, locked, windows-latest)	2m 25s
checks / test (3.14, lowest-direct, windows-latest)	2m 54s
checks / test (3.12, lowest-direct, windows-latest)	4m 4s
checks / test (3.13, locked, ubuntu-latest)	45s
all-green	5s
	42m 18s

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