Update README section on Python3 support

Now that we have a gating Python 3 job running on all tempest commits we
can update the language in the Readme about the Python 3 section. This
commit does just that.

Change-Id: I2a1d35cae6c28a0b427ffd142b8a3d2f25a6dbe7
diff --git a/README.rst b/README.rst
index 9d19c23..3d7c804 100644
--- a/README.rst
+++ b/README.rst
@@ -209,15 +209,9 @@
 Python 3.x
 ----------
 
-Starting during the Liberty release development cycle work began on enabling
-Tempest to run under both Python 2.7 and Python 3.4. Tempest strives to fully
-support running with Python 3.4 and newer. A gating unit test job was added to
-also run Tempest's unit tests under Python 3. This means that the Tempest
-code at least imports under Python 3.4 and things that have unit test coverage
-will work on Python 3.4. However, because large parts of Tempest are
-self-verifying there might be uncaught issues running on Python 3. So until
-there is a gating job which does a full Tempest run using Python 3 there
-isn't any guarantee that running Tempest under Python 3 is bug free.
+Starting during the Pike cycle Tempest has a gating CI job that runs tempest
+with Python 3. Any tempest release after 15.0.0 should fully support running
+under Python 3 as well as Python 2.7.
 
 Legacy run method
 -----------------