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: I9f540025c9d57d3c6eb27b0d3821b0ba2329de52

test-requirements.txt

@@ -16,7 +16,7 @@ requests>=2.14.2  # Apache-2.0
 requestsexceptions>=1.2.0 # Apache-2.0
 lxml>=2.3,!=3.7.0  # BSD
 PyYAML>=3.10.0  # MIT
-jsonschema>=2.6.0
+jsonschema>=3.2.0
 os-service-types
 openstack-governance>=0.1.0
 percache>=0.3.0