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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def session(self):
 """ http session interface, transparent proxy to requests.session """

        if self._session is None:
            self._session = requests.session()
            self._session.headers.update(
                {
                    "Authorization": "Bearer {}".format(self.token),
                    "User-Agent": "ads-api-client/{}".format(__version__),
                    "Content-Type": "application/json",
                }
            )
        return self._session