ci: add Slack notification for maintainer PRs #938

Job	Run time
checks / readme-snippets	12s
checks / pre-commit	31s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.12, lowest-direct, windows-latest)	3m 45s
checks / test (3.10, lowest-direct, windows-latest)	2m 39s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
checks / test (3.12, locked, windows-latest)	3m 32s
checks / test (3.11, locked, windows-latest)	2m 48s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.14, lowest-direct, windows-latest)	2m 22s
checks / test (3.11, lowest-direct, ubuntu-latest)	58s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 15s
checks / test (3.13, locked, ubuntu-latest)	52s
checks / test (3.10, locked, windows-latest)	2m 23s
checks / test (3.14, locked, ubuntu-latest)	2m 22s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.13, locked, windows-latest)	2m 18s
checks / test (3.11, lowest-direct, windows-latest)	2m 45s
checks / test (3.14, locked, windows-latest)	3m 51s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.13, lowest-direct, windows-latest)	2m 41s
all-green	4s
Notify maintainers	4s
	40m 49s

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