update constraint for mistral-lib to new release 3.1.0

meta: version: 3.1.0
meta: team: mistral
meta: diff-start: -
meta: series: epoxy
meta: branch: master
meta: release-type: release
meta: pypi: yes
meta: first: yes
meta: release:Author: Elod Illes <elod.illes@est.tech>
meta: release:Commit: Elod Illes <elod.illes@est.tech>
meta: release:Change-Id: Ic4a1912a2a4aec4bf6b6a56d7274c98728978806
meta: release:Code-Review+1: foundjem <foundjem@ieee.org>
meta: release:Workflow+1: Thierry Carrez <thierry@openstack.org>
meta: release:Code-Review+2: Elod Illes <elod.illes@est.tech>
meta: release:Code-Review+2: Thierry Carrez <thierry@openstack.org>
Change-Id: Ib74b166453feab357e7309e073d08c0365fbcde7

Author: OpenStack Proposal Bot

upper-constraints.txt

@@ -294,7 +294,7 @@ threadpoolctl===3.5.0
 os-ken===2.11.2
 ujson===5.10.0
 selenium===3.141.0
-mistral-lib===3.0.0
+mistral-lib===3.1.0
 dogtag-pki===11.2.1
 XStatic-Angular-UUID===0.0.4.0
 purestorage===1.19.0