Cap jsonschema 3.2.0 as the minimal version

Previous versions of jsonschema (<3.2.0) doesn't support python 3.8 [1].
Python 3.8 is part of the victoria supported runtimes [2] so we now force
to use jsonschema version 3.2.0 to avoid issues, remove ambiguity and ensure
that everything works with python 3 in general.

[1] https://github.com/Julian/jsonschema/pull/627
[2] https://governance.openstack.org/tc/reference/runtimes/victoria.html#python-runtimes-for-victoria

Change-Id: Id476227552c3fa91eecadbc6c4370c354f56a40d

lower-constraints.txt

@@ -42,7 +42,7 @@ Jinja2==2.10
 jmespath==0.9.3
 jsonpatch==1.21
 jsonpointer==2.0
-jsonschema==2.6.0
+jsonschema==3.2.0
 keystoneauth1==3.4.0
 keystonemiddleware==4.21.0
 kombu==4.1.0