Adapt to flake8-import-order

https://review.openstack.org/#/c/432906/
flake8-import-order adds 3 new flake8 warnings:
I100: Your import statements are in the wrong order.
I101: The names in your from import are in the wrong order.
I201: Missing newline between sections or imports.

Signed-off-by: Ben Pfaff <blp@ovn.org>

python/ovs/json.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 from __future__ import absolute_import
+
 import functools
 import json
 import re