Introduce flake8-import-order

This commit introduces flake8-import-order to check import orders
automatically. We shouldn't do it manually.

Depends-On: Ia2140f5566ce169b076ffa5c9ebfbdbfc41e5bed
Change-Id: Iff75ebec42b295870edd1c04001adfb17460a682
diff --git a/tox.ini b/tox.ini
index 46823d8..d8d390e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -147,6 +147,7 @@
 show-source = True
 exclude = .git,.venv,.tox,dist,doc,*egg
 enable-extensions = H106,H203,H904
+import-order-style = pep8
 
 [testenv:releasenotes]
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html