Release oslo.serialization 3.0.0

This version officially drop the python 2 support so we release a major
version of oslo.serialization to inform user that we break
the backward compatibility.

OpenStack is dropping the py2.7 support in ussuri cycle.

Complete discussion & schedule can be found in
-
http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I02395f920e39a39a013004b0e664f76e20e60665

deliverables/ussuri/oslo.serialization.yaml

@@ -6,3 +6,8 @@ team: oslo
 type: library
 repository-settings:
   openstack/oslo.serialization: {}
+releases:
+  - version: 3.0.0
+    projects:
+      - repo: openstack/oslo.serialization
+        hash: 9e4adf56b22c8005af1722815ea4c4bfc03fb12d