Per best practices, explicitly use the python3 executable rather than
assuming the platform with have a "python" executable that maps to
python3.
https://www.python.org/dev/peps/pep-0394/
Change-Id: I39b8a1013a891f4570f374d8faa3cfa2ecaf3347
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>