Remove six usage
Remove six usage

This repo does not support Python 2 anymore, so we don't need
six for compatibility between Python2 and 3, convert six usage to Python
3 code.

Note that six.text_type is "str" in Python 3, so we can remove the
clause completly.

Needed-By: https://review.opendev.org/701743
Change-Id: I7dbdf7b773d72db9204c6251b1ce561cf7675522

openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui