Bump pyvoltha to 0.2.3

Includes fixes to omci timing

Change-Id: I2b35b6dd11b3c3af3002460183ca76805514e96d
diff --git a/python/requirements.txt b/python/requirements.txt
index 556e5e3..4284338 100644
--- a/python/requirements.txt
+++ b/python/requirements.txt
@@ -60,4 +60,4 @@
 python-consul==0.6.2
 afkak==3.0.0.dev20181106
 voltha-protos>=0.1.4
-pyvoltha==0.2.2
+pyvoltha==0.2.3