Use python-six shim for assertRaisesRegex/p

Python2 uses assertRaisesRegexp while Python3 uses assertRaisesRegex.
Use the compatability shim in six for this:
https://pythonhosted.org/six/#unittest-assertions

Change-Id: I28ce94207567e0b3c28c634119757a1d68e955f2
Closes-Bug: #1462370

keystoneclient/tests/unit/test_session.py

@@ -244,7 +244,8 @@ class SessionTests(utils.TestCase):
         # The exception should contain the URL and details about the SSL error
         msg = _('SSL exception connecting to %(url)s: %(error)s') % {
             'url': self.TEST_URL, 'error': error}
-        self.assertRaisesRegex(exceptions.SSLError,
+        six.assertRaisesRegex(self,
+                              exceptions.SSLError,
                               msg,
                               session.get,
                               self.TEST_URL)