After user has authorized the request token, get access token
        with user supplied verifier.

def get_access_token(self, verifier=None):
        """
        After user has authorized the request token, get access token
        with user supplied verifier.
        """
        try:
            url = self._get_oauth_url('access_token')
            self.oauth = OAuth1Session(self.consumer_key,
                                       client_secret=self.consumer_secret,
                                       resource_owner_key=self.request_token['oauth_token'],
                                       resource_owner_secret=self.request_token['oauth_token_secret'],
                                       verifier=verifier, callback_uri=self.callback)
            resp = self.oauth.fetch_access_token(url)
            self.access_token = resp['oauth_token']
            self.access_token_secret = resp['oauth_token_secret']
            return self.access_token, self.access_token_secret
        except Exception as e:
            raise TweepError(e)