Add Python 3.15 test infrastructure #2457

Job	Run time
stubtest: third party (ubuntu-latest, 2)	7m 11s
stubtest: third party (ubuntu-latest, 0)	5m 49s
stubtest: third party (windows-latest, 1)	1m 43s
stubtest: third party (windows-latest, 0)	2m 16s
stub_uploader tests	5m 12s
stubtest: third party (windows-latest, 3)	1m 25s
stubtest: third party (macos-latest, 2)	39s
stubtest: third party (ubuntu-latest, 1)	6m 17s
stubtest: third party (windows-latest, 2)	1m 50s
stubtest: third party (ubuntu-latest, 3)	9m 31s
stubtest: stdlib (ubuntu-latest, 3.12)	36s
stubtest: stdlib (ubuntu-latest, 3.10)	44s
stubtest: third party (macos-latest, 1)	1m 25s
stubtest: stdlib (windows-latest, 3.11)	1m 25s
stubtest: stdlib (ubuntu-latest, 3.15)	50s
stubtest: stdlib (windows-latest, 3.13)	1m 57s
stubtest: stdlib (macos-latest, 3.11)	27s
stubtest: stdlib (macos-latest, 3.14)	29s
stubtest: third party (macos-latest, 3)	1m 11s
stubtest: stdlib (macos-latest, 3.13)	30s
stubtest: stdlib (macos-latest, 3.15)	50s
stubtest: stdlib (ubuntu-latest, 3.13)	33s
stubtest: stdlib (macos-latest, 3.10)	51s
stubtest: stdlib (macos-latest, 3.12)	30s
stubtest: stdlib (ubuntu-latest, 3.14)	42s
stubtest: stdlib (windows-latest, 3.14)	1m 12s
stubtest: stdlib (ubuntu-latest, 3.11)	37s
stubtest: stdlib (windows-latest, 3.12)	1m 14s
stubtest: stdlib (windows-latest, 3.15)	1m 53s
stubtest: third party (macos-latest, 0)	1m 1s
Create issue on failure	0s
	1h 0m 50s

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