Fix flake8 upgrade.
# It's much more strict than our check for lint across the entire code base.
set -e
-flake8 --version | grep '^3\.6\.' >/dev/null || \
+flake8 --version | grep '^3\.[6-9]\.' >/dev/null || \
( echo "Please install flake8 >=3.6.0." && false )
CHANGED_FILES="$(git diff --name-only master... | grep '\.py$' | tr '\n' ' ')"
