Update flake8 requirement for 0.11.0
Update flake8 requirement for 0.11.0

We need to update our flake8 dependency version so we can use
--allow-extensions to be used with our off-by-default rules.

Change-Id: I3171244a14ebc110f2b041b9db75499f007f472b
openstack-dev/hacking,openstack-dev/hacking