Add flake8-import-order extension to tox.ini

This package 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.

Change-Id: I4feabcd4db583d213e0e4d414eee267b0ddb8870
diff --git a/test-requirements.txt b/test-requirements.txt
index f230fa9..cf025c3 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -13,3 +13,4 @@
 openstackdocstheme>=1.31.2 # Apache-2.0
 # releasenotes
 reno>=2.5.0 # Apache-2.0
+flake8-import-order