Drop support for Python 3.9 #1693

Job	Run time
service-tests (buildbarn)	1m 47s
Build Python wheel packages on ubuntu-24.04	5m 4s
service-tests (buildgrid)	3m 38s
build_docs	4m 26s
tests (mypy)	45s
tests (fedora-43)	28m 20s
tests (fedora-missing-deps)	13m 39s
tests (ubuntu-22.04)	32m 10s
tests (debian-13)	28m 14s
tests (debian-12)	28m 31s
tests (lint)	2m 3s
tests (fedora-42)	25m 33s
tests (plugins-master, true)	25m 27s
Test Python packages: wheels-manylinux_2_28-cp311	1m 27s
Test Python packages: wheels-manylinux_2_28-cp310	1m 33s
Test Python packages: wheels-manylinux_2_28-cp313	1m 36s
Test Python packages: wheels-manylinux_2_28-cp312	1m 39s
Test Python packages: wheels-manylinux_2_28-cp314	2m 10s
	3h 28m 2s

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