Add support for Python 3.14 #1661

Job	Run time
Build Python wheel packages on ubuntu-24.04	4m 1s
tests (debian-12)	4m 1s
build_docs	1m 39s
service-tests (buildbarn)	1m 45s
service-tests (buildgrid)	1m 59s
tests (ubuntu-22.04)	4m 0s
tests (debian-13)	4m 1s
tests (debian-11)	4m 1s
tests (plugins-master, true)	4m 1s
tests (fedora-missing-deps)	4m 1s
tests (fedora-42)	4m 3s
tests (lint)	2m 10s
tests (mypy)	43s
Test Python packages: ${{ matrix.test-name }}	1s
	40m 26s

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