Remove deprecated --experimental option of autopep8

This option was deprecated in v2.2.0[1].

[1] https://github.com/hhatto/autopep8/issues/422

Change-Id: I8be7fe4082f2f2630b492e469fa380bae04d2d03
diff --git a/tox.ini b/tox.ini
index 0fbc252..b8ba32b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -389,7 +389,7 @@
     {[testenv]deps}
     autopep8>=2.1.0
 commands =
-    autopep8 --exit-code --max-line-length=79 --experimental --diff -r tempest setup.py
+    autopep8 --exit-code --max-line-length=79 --diff -r tempest setup.py
     flake8 {posargs}
     check-uuid