Upgrade build.py to python 3 #2112
Description
build.py currently requires Python 2.
We should support python 3, while still working for either. Developers using python 2 should not see a breakage when we add a python 3 version.
This can be a separate script if that's easiest.
We also need a deprecation warning in the current script, and possibly a change to the developer documentation. The developer documentation change needs to be done by a Blockly team member and can be done separately once we have the new script.