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I can only download usdpython from following website: https://developer.apple.com/augmented-reality/tools/ And where could I get the various versions of the usdpython? Is the usdzconvert(usdpython) of Apple open source? Because I wanna learn how Apple achieves the conversion from GLTF to USDZ because I'm currently using version 0.66, and I feel that the conversion of GLTF features is not quite sufficient.