def _getContextFactory(self):
        """
        Get a context factory with which to negotiate TLS.

        @rtype: L{None} or
            L{ClientContextFactory <twisted.internet.ssl.ClientContextFactory>}
        @return: A context factory or L{None} if TLS is not supported on the
            client.
        """
        try:
            from twisted.internet import ssl
        except ImportError:
            return None
        else:
            context = ssl.ClientContextFactory()
            context.method = ssl.SSL.TLSv1_METHOD
            return context

        Get a context factory with which to negotiate TLS.

        @rtype: L{None} or
            L{ClientContextFactory <twisted.internet.ssl.ClientContextFactory>}
        @return: A context factory or L{None} if TLS is not supported on the
            client.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/pop3client.py