Use except x as y instead of except x, y

According to https://docs.python.org/3/howto/pyporting.html the
syntax changed in Python 3.x. The new syntax is usable with
Python >= 2.6 and should be preferred to be compatible with Python3.

Enabled hacking check H231.

Change-Id: I7ce80163cc328c0368583d1dc76e9e0b0f7cd6dc

tests/test_layoutvalidator.py

@@ -44,7 +44,7 @@ class TestLayoutValidator(testtools.TestCase):
             if m.group(1) == 'good':
                 try:
                     validator.validate(data)
-                except voluptuous.Invalid, e:
+                except voluptuous.Invalid as e:
                     raise Exception(
                         'Unexpected YAML syntax error in %s:\n  %s' %
                         (fn, str(e)))
@@ -53,7 +53,7 @@ class TestLayoutValidator(testtools.TestCase):
                     validator.validate(data)
                     raise Exception("Expected a YAML syntax error in %s." %
                                     fn)
-                except voluptuous.Invalid, e:
+                except voluptuous.Invalid as e:
                     error = str(e)
                     print '  ', error
                     if error in errors:

tox.ini

@@ -37,5 +37,6 @@ commands = zuul-server -c etc/zuul.conf-sample -t -l {posargs}
 
 [flake8]
 ignore = E125,H
+select = H231
 show-source = True
 exclude = .venv,.tox,dist,doc,build,*.egg