Reroute assertRaisesRegexp to prevent Python3 deprecation undermining warnings tests.

Author: pp-mo

lib/iris/tests/__init__.py

@@ -512,6 +512,17 @@ def assertXMLElement(self, obj, reference_filename):
     def assertArrayEqual(self, a, b, err_msg=''):
         np.testing.assert_array_equal(a, b, err_msg=err_msg)
 
+    def assertRaisesRegexp(self, *args, **kwargs):
+        """
+        Emulate the old :meth:`unittest.TestCase.assertRaisesRegexp`.
+
+        Because the original function is now deprecated in Python 3.
+        Now calls :meth:`six.assertRaisesRegex()` (no final "p") instead.
+        It is the same, except for providing an additional 'msg' argument.
+
+        """
+        return six.assertRaisesRegex(self, *args, **kwargs)
+
     def _assertMaskedArray(self, assertion, a, b, strict, **kwargs):
         # Define helper function to extract unmasked values as a 1d
         # array.