Uncap jsonschema

We have jsonschema capped at a fairly old version. Other than some
specific releases, it looks like keeping it below 3.0 was added in
I943fd68b9fab3bce1764305a5058df5339470757 without really any explanation
why.

In order to update to a 3.x release we need to:
1. Remove the cap from global-requirements.txt (see Depends-On), leaving
   upper-constraints.txt at a 2.x release
2. Remove the cap from all consumers (this change)
3. Release a new version of consumers that are published to pypi
4. Update upper-constraints.txt with those new releases
5. Update jsonschema in upper-constraints.txt to a 3.X release
   (See: https://review.openstack.org/649789)
6. Test consumers with the change from 5.
7. [Optional] fix issues in consumers that arise from 6.
8. Merge the change from 5.

Change-Id: I95b5520ca0e8da7b2a7663a272bc740b8c809592
Co-Authored-by: Sean McGinnis <sean.mcginnis@gmail.com>
Depends-On: https://review.openstack.org/649669

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<3.0.0,>=2.6.0 # MIT
+jsonschema>=2.6.0 # MIT
 testtools>=2.2.0 # MIT
 paramiko>=2.0.0 # LGPLv2.1+
 netaddr>=0.7.18 # BSD