Re-use testenv deps in pep8

followup to I867bec362d83ab820aa6b11dc4770d50133ecdbf

otherwise, when tox installs tempest itself, it will install
its dependecies w/o using constraints file

Change-Id: I7f7b773179f8b3f13213e1cb2249e7dd672e77da
Related-Issue: PRODX-3456
diff --git a/tox.ini b/tox.ini
index 6fa7faf..d2e2288 100644
--- a/tox.ini
+++ b/tox.ini
@@ -409,8 +409,7 @@
 
 [testenv:pep8]
 deps =
-    -r{toxinidir}/test-requirements.txt
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    {[testenv]deps}
     autopep8
 commands =
     autopep8 --exit-code --max-line-length=79 --experimental --diff -r tempest setup.py