Use python3 in various python scripts am: c922835189 am: 5e0921709b am: ee655c3444 am: 039e6a7b88

Original change: https://android-review.googlesource.com/c/platform/external/icu/+/1784621

Change-Id: Iecf7b30ca8532741e8a3c6f50035726684cb3d97