retrieve access token with code fetched via 
        retrieve_authorization_code method.

def retrieve_token(self):
        """ retrieve access token with code fetched via 
        retrieve_authorization_code method.
        """

        if self.authorization_code:
            request_param = {
                "client_id": self.client_id,
                "client_secret": self.client_secret,
                "redirect_uri": self.redirect_uri,
                "code": self.authorization_code
                }

            if self._extra_token_params:
                request_param.update(self._extra_token_params)

            content_length = len(urlencode(request_param))
            headers = {
                'Content-Length': str(content_length),
                'Content-Type': 'application/x-www-form-urlencoded'
                }
        
            r = requests.post(self.token_uri, data=request_param,
                              headers=headers)
            jsondata = json.loads(r.text)
            self.access_token = jsondata
            return self.access_token

        else:
            print("authorization code is required before getting accesss token")
            print("Please call retrieve_authorization_code() beforehand")