Migrate to Python 3

Python 2 will not be maintained past April 2020. Fortunately our scripts
require almost no changes to continue working under Python 3.

This patch forces the use of `python3` binary for all `action*` build
targets in GN and updates hashbang directives in our scripts.

Change-Id: Ic34ac547dfe5f85023ddcf4d1a969818f94a72fb
12 files changed