Older hacking version depends on pep8, but pep8 module is replaced
by pycodestyle. This is already resolved by updating hacking
version in https://review.openstack.org/#/c/627149/
pep8 is dropped, so no longer require in lower-constraints.txt.
According to Openstack summit session ,
stestr is maintained project to which all Openstack projects should migrate.
Let's switch to stestr as other projects have already moved to it.