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/.gitignore b/.gitignore
index a6a0b0b..8c8997c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,7 +14,7 @@
 *.log
 .coverage
 .coverage.*
-/.testrepository/*
+.stestr/
 .tox
 .venv
 cover