ci: add Slack notification for maintainer PRs #940

Job	Run time
checks / pre-commit	41s
checks / readme-snippets	12s
checks / test (3.11, lowest-direct, windows-latest)	2m 29s
checks / test (3.10, locked, windows-latest)	2m 12s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.11, lowest-direct, ubuntu-latest)	53s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, lowest-direct, windows-latest)	2m 26s
checks / test (3.12, lowest-direct, windows-latest)	4m 10s
checks / test (3.11, locked, windows-latest)	2m 46s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 5s
checks / test (3.13, lowest-direct, windows-latest)	2m 31s
checks / test (3.13, locked, windows-latest)	2m 24s
checks / test (3.12, locked, windows-latest)	3m 57s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 24s
checks / test (3.14, locked, ubuntu-latest)	2m 25s
checks / test (3.14, locked, windows-latest)	3m 10s
checks / test (3.14, lowest-direct, windows-latest)	2m 27s
all-green	3s
	40m 25s

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