Add Python 3.15 test infrastructure #2465

Job	Run time
stubtest: third party (macos-latest, 1)	1m 8s
stubtest: third party (ubuntu-latest, 3)	10m 9s
stubtest: third party (ubuntu-latest, 2)	6m 12s
stubtest: third party (windows-latest, 0)	1m 27s
stubtest: third party (windows-latest, 1)	1m 48s
stubtest: third party (windows-latest, 3)	1m 54s
stubtest: third party (macos-latest, 2)	33s
stubtest: stdlib (windows-latest, 3.11)	1m 39s
stubtest: stdlib (macos-latest, 3.10)	58s
stubtest: third party (macos-latest, 0)	1m 2s
stubtest: stdlib (ubuntu-latest, 3.15)	51s
stubtest: stdlib (ubuntu-latest, 3.12)	32s
stubtest: stdlib (macos-latest, 3.11)	41s
stubtest: stdlib (windows-latest, 3.14)	1m 14s
stubtest: stdlib (ubuntu-latest, 3.10)	36s
stubtest: stdlib (ubuntu-latest, 3.11)	36s
stubtest: stdlib (windows-latest, 3.15)	2m 10s
stubtest: stdlib (macos-latest, 3.13)	23s
stubtest: stdlib (ubuntu-latest, 3.14)	55s
stubtest: third party (ubuntu-latest, 1)	5m 51s
stubtest: stdlib (macos-latest, 3.12)	28s
stubtest: third party (windows-latest, 2)	2m 27s
stubtest: stdlib (windows-latest, 3.12)	1m 25s
stubtest: stdlib (ubuntu-latest, 3.13)	46s
stubtest: third party (ubuntu-latest, 0)	6m 23s
stubtest: stdlib (windows-latest, 3.13)	1m 2s
stubtest: stdlib (macos-latest, 3.14)	23s
stubtest: stdlib (macos-latest, 3.15)	45s
stubtest: third party (macos-latest, 3)	54s
stub_uploader tests	5m 28s
Create issue on failure	0s
	1h 0m 40s

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