Currently, many of the Python scripts in the engine repo, and some of our dependencies, like the Dart SDK, rely on Python 2.7 which was officially end-of-lifed in 2020. We should update our tooling to work with both Python 2.7 and Python 3.

Platform verification:

• Ninja file generation (tools/gn) (Fix: Make tools/gn compatible with Python 3 engine#26321)
• Linux x86_64 (Fix: Update sysroot_ld_path.py for Python3 buildroot#461)
• Linux arm64
• Windows win32
• Windows UWP
• Fuchsia
• macOS (Fix: Python 3 support macOS/iOS builds buildroot#463)
• iOS armv7, arm64, lipo script
• iOS simulator
• Android armv7 (Fix: Python 3 migration required for Android builds buildroot#462)
• Android arm64 (Fix: Python 3 migration required for Android builds buildroot#462)
• Android x86 (Fix: Python 3 migration required for Android builds buildroot#462)
• Flip script_executable to "python3" in .gn in buildroot
• Migrate all remaining #! lines in scripts from python to python3