title: test error message in python 3
body: <p>I have the following test method</p>
  
  <pre><code>def test_fingerprintBadFormat(self):
      """
      A C{BadFingerPrintFormat} error is raised when unsupported
      formats are requested.
      """
      with self.assertRaises(keys.BadFingerPrintFormat) as em:
          keys.Key(self.rsaObj).fingerprint('sha256-base')
      self.assertEqual('Unsupported fingerprint format: sha256-base',
          em.exception.message)
  </code></pre>
  
  <p>Here is the exception class.</p>
  
  <pre><code>class BadFingerPrintFormat(Exception):
      """
      Raises when unsupported fingerprint formats are presented to fingerprint.
      """
  </code></pre>
  
  <p>this test method works perfectly fine in Python2 but fails in python 3 with the following message</p>
  
  <pre><code>builtins.AttributeError: 'BadFingerPrintFormat' object has no attribute 'message'
  </code></pre>
  
  <p>How can I test the error message in Python3. I don't like the idea of using <code>asserRaisesRegex</code> as it tests the regex rather than the exception message.</p>