update constraint for oslo.serialization to new release 2.18.0

Change-Id: If79335411401cfdec008450365863a82695d66ad
meta:version: 2.18.0
meta:diff-start: -
meta:series: pike
meta:release-type: release
meta:pypi: yes
meta:first: no
meta:release:Author: ChangBo Guo(gcb) <eric.guo@easystack.cn>
meta:release:Commit: ChangBo Guo(gcb) <eric.guo@easystack.cn>
meta:release:Change-Id: I3c70beb0024527ebf978ba5110c089a4f99570b2
meta:release:Code-Review+2: Thierry Carrez <thierry@openstack.org>
meta:release:Code-Review+2: Davanum Srinivas (dims) <davanum@gmail.com>
meta:release:Workflow+1: Davanum Srinivas (dims) <davanum@gmail.com>

upper-constraints.txt

@@ -254,7 +254,7 @@ pydotplus===2.0.2
 urwid===1.3.1;python_version=='3.4'
 urwid===1.3.1;python_version=='3.5'
 singledispatch===3.4.0.3;python_version=='2.7'
-oslo.serialization===2.17.0
+oslo.serialization===2.18.0
 warlock===1.2.0
 sphinxcontrib-httpdomain===1.5.0
 murano-pkg-check===0.3.0