Switch to use stestr for unit test

According to Openstack summit session [1],
stestr is maintained project to which all Openstack projects should migrate.
So we should switch to stestr.

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

Change-Id: I6c6e1b3873a71d3b069f0c0bc45d9ba9885c2886
diff --git a/test-requirements.txt b/test-requirements.txt
index 91ee374..8a78edb 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -9,6 +9,6 @@
 sphinx>=1.6.2 # BSD
 oslosphinx>=4.7.0 # Apache-2.0
 oslotest>=1.10.0 # Apache-2.0
-testrepository>=0.0.18 # Apache-2.0/BSD
+stestr>=2.0.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT