ci: add Slack notification for maintainer PRs #936

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	31s
checks / test (3.10, locked, windows-latest)	2m 27s
checks / test (3.11, lowest-direct, ubuntu-latest)	53s
checks / test (3.11, lowest-direct, windows-latest)	2m 44s
checks / test (3.11, locked, ubuntu-latest)	48s
checks / test (3.10, lowest-direct, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.10, locked, ubuntu-latest)	57s
checks / test (3.11, locked, windows-latest)	2m 54s
checks / test (3.14, locked, ubuntu-latest)	2m 19s
checks / test (3.12, lowest-direct, windows-latest)	3m 30s
checks / test (3.10, lowest-direct, windows-latest)	2m 32s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.13, lowest-direct, ubuntu-latest)	54s
checks / test (3.14, lowest-direct, windows-latest)	2m 24s
checks / test (3.13, lowest-direct, windows-latest)	2m 37s
checks / test (3.13, locked, windows-latest)	2m 19s
checks / test (3.14, locked, windows-latest)	3m 1s
checks / test (3.12, locked, windows-latest)	3m 21s
all-green	5s
Notify maintainers	2s
	39m 27s

Provide feedback