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/.gitignore b/.gitignore
index 5685b36..48f6897 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,7 +6,7 @@
 *.sqlite
 *.swp
 .coverage
-.testrepository
+.stestr/
 .tox
 .venv
 .idea