According to Openstack summit session [1] stestr is
maintained project to which all Openstack projects
should migrate.
Let's switch it then.
Also clean up requirements.
[1] https://etherpad.openstack.org/p/YVR-python-pti
Change-Id: Ic9317d0d2e71ffcc5012184510b554453a7e322f
Signed-off-by: Charles Short <zulcss@gmail.com>