gh-117398: Add multiphase support to _datetime #77

Job	Run time
Free-Threaded (Debug)	10m 16s
i686-pc-windows-msvc/msvc (Debug)	18m 50s
i686-pc-windows-msvc/msvc (Release)	23m 53s
x86_64-pc-windows-msvc/msvc (Debug)	20m 48s
x86_64-pc-windows-msvc/msvc (Release)	22m 6s
aarch64-pc-windows-msvc/msvc (Debug)	5m 16s
aarch64-pc-windows-msvc/msvc (Release)	6m 20s
x86_64-apple-darwin/clang (Debug)	14m 11s
x86_64-apple-darwin/clang (Release)	19m 29s
aarch64-apple-darwin/clang (Debug)	10m 36s
aarch64-apple-darwin/clang (Release)	15m 44s
x86_64-unknown-linux-gnu/gcc (Debug)	8m 28s
x86_64-unknown-linux-gnu/gcc (Release)	11m 13s
x86_64-unknown-linux-gnu/clang (Debug)	8m 29s
x86_64-unknown-linux-gnu/clang (Release)	11m 26s
aarch64-unknown-linux-gnu/gcc (Debug)	1h 10m 51s
aarch64-unknown-linux-gnu/gcc (Release)	48m 35s
aarch64-unknown-linux-gnu/clang (Debug)	1h 2m 49s
aarch64-unknown-linux-gnu/clang (Release)	36m 3s
	7h 5m 23s

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