Bump to Python 3.14.0rc3. #264

Job	Run time
config	20s
build (macOS)	53s
build (watchOS)	3m 28s
build (iOS)	2m 39s
build (tvOS)	1m 55s
build (visionOS)	2m 37s
CPython testbed (${{ matrix.platform }})	0s
Briefcase testbed (${{ matrix.platform }})	0s
Cross-platform env test (${{ matrix.multiarch }})	0s
build (watchOS)	5m 52s
build (tvOS)	6m 2s
build (iOS)	9m 12s
build (macOS)	53s
build (visionOS)	5m 32s
config	20s
Briefcase testbed (iOS)	4m 53s
Briefcase testbed (macOS)	1m 30s
CPython testbed (iOS)	5m 0s
Cross-platform env test (x86_64-iphonesimulator)	38s
CPython testbed (visionOS)	2m 27s
Cross-platform env test (arm64-iphonesimulator)	38s
Cross-platform env test (arm64-iphoneos)	32s
	55m 21s

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