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: I7eaadde83243afed1929bd29fff63272e58e072b
diff --git a/tox.ini b/tox.ini
index f183956..4d9c7df 100644
--- a/tox.ini
+++ b/tox.ini
@@ -18,7 +18,7 @@
                       rm
 commands =
   find . -type f -name "*.pyc" -delete
-  sh tools/pretty_tox.sh '{posargs}'
+  stestr run {posargs}
 passenv = http_proxy
           HTTP_PROXY
           https_proxy