If a valid access token is in memory, returns it
        Else feches a new token and returns it

def get_access_token(self):
        """
        If a valid access token is in memory, returns it
        Else feches a new token and returns it
        """
        if self.token_info and not self.is_token_expired(self.token_info):
            return self.token_info['access_token']

        token_info = self._request_access_token()
        token_info = self._add_custom_values_to_token_info(token_info)
        self.token_info = token_info
        return self.token_info['access_token']