Use upper constraints in pep8 test run

this is to avoid current problem due to
https://github.com/mtreinish/stestr/issues/347
but also to equalize dependencies with other test  envs.

Change-Id: I867bec362d83ab820aa6b11dc4770d50133ecdbf
diff --git a/tox.ini b/tox.ini
index fc882cf..e809f1c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -384,7 +384,7 @@
 
 [testenv:pep8]
 deps =
-    -r{toxinidir}/test-requirements.txt
+    {[testenv]deps}
     autopep8
 commands =
     autopep8 --exit-code --max-line-length=79 --experimental --diff -r tempest setup.py