Switch to stestr

According to Openstack summit session [1],
stestr is maintained project to which all Openstack projects should migrate.
Let's switch to stestr as other projects have already moved to it.

[1] https://etherpad.openstack.org/p/YVR-python-pti

Change-Id: I3fb32622b029bfadef7f34e973d8277c107112f0
diff --git a/tox.ini b/tox.ini
index 1f1ce01..6fec391 100644
--- a/tox.ini
+++ b/tox.ini
@@ -10,7 +10,7 @@
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
 deps = -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --slowest --testr-args='{posargs}'
+commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
 commands =