Automatically add backport labels for release branch #900

Job	Run time
check-commits	8s
build / build	12s
lint / lint (3.11)	36s
lint / lint (3.12)	36s
docs / test	1m 3s
test / test (nightly, /relocated/djnd/, true, 3.11)	16m 16s
docs / no-test	0s
test / test (latest, 3.11)	16m 40s
codeql / Analyze	1m 40s
test / test (3.39, 3.12)	15m 16s
test / test (3.28, true, 3.12)	14m 57s
test / test (3.22, 3.8)	12m 35s
test / test (3.21, /relocated/djnd/, 3.9)	12m 30s
test / test (3.18, true, 3.10)	12m 0s
ready-to-ship	3s
	1h 44m 32s

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