Pin flake8 requirement to version 3.7.9
Pin flake8 requirement to version 3.7.9

This version shouldn't complain with code E741 about ambiguous variable
names in a couple of our list comprehensions

ministryofjustice/cla_public,ministryofjustice/cla_public,ministryofjustice/cla_public,ministryofjustice/cla_public