Add Python 3.15 test infrastructure #15153

Job	Run time
stubtest: stdlib (windows-latest, 3.12)	1m 8s
stubtest: stdlib (windows-latest, 3.14)	1m 10s
stubtest: stdlib (ubuntu-latest, 3.10)	43s
stubtest: stdlib (ubuntu-latest, 3.11)	36s
stubtest: stdlib (ubuntu-latest, 3.14)	37s
stubtest: stdlib (windows-latest, 3.13)	1m 24s
stubtest: stdlib (macos-latest, 3.12)	27s
stubtest: stdlib (macos-latest, 3.13)	31s
stubtest: stdlib (macos-latest, 3.14)	27s
stubtest: stdlib (macos-latest, 3.15)	53s
stubtest: stdlib (windows-latest, 3.11)	1m 31s
stubtest: stdlib (windows-latest, 3.15)	1m 21s
stubtest: stdlib (macos-latest, 3.10)	56s
stubtest: stdlib (macos-latest, 3.11)	31s
stubtest: stdlib (ubuntu-latest, 3.13)	37s
stubtest: stdlib (ubuntu-latest, 3.15)	51s
stubtest: stdlib (ubuntu-latest, 3.12)	31s
	14m 14s

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