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: Id2c662c0d6146423855714a0f240eed682e3c004
diff --git a/requirements.txt b/requirements.txt
index bf38fae..d8738f0 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,7 +3,7 @@
 # process, which may cause wedges in the gate later.
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 cliff!=2.9.0,>=2.8.0 # Apache-2.0
-jsonschema>=2.6.0 # MIT
+jsonschema>=3.2.0 # MIT
 testtools>=2.2.0 # MIT
 paramiko>=2.0.0 # LGPLv2.1+
 netaddr>=0.7.18 # BSD