'L{_checkRequirements} raises L{ImportError} when run on a version of
Python that is too old.'

def test_oldPython(self):

    sys.version_info = self.unsupportedPythonVersion
    with self.assertRaises(ImportError) as raised:
        _checkRequirements()
    self.assertEqual(('Twisted    requires    Python    %d.%d    or    later.' % self.supportedPythonVersion), str(raised.exception))
