Update all scripts to use python3 instead of python.
Review Request #11834 — Created Oct. 2, 2021 and submitted
This updates all scripts in the Djblets tree to use
python, ensuring that the right version is used when in an environment
containing both versions.
run-pyflakes.pyneeded some changes to run on Python 3. While we may
want to remove this script, that's a decision that'll be left for after
Tested all scripts and made sure they ran.