<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_oauth_token(self):
 """ Get Monzo access token via OAuth2 `authorization code` grant type. Official docs: https://monzo.com/docs/#acquire-an-access-token :returns: OAuth 2 access token :rtype: dict """

        url = urljoin(self.api_url, '/oauth2/token')

        oauth = OAuth2Session(
            client_id=self._client_id,
            redirect_uri=config.REDIRECT_URI,
        )

        token = oauth.fetch_token(
            token_url=url,
            code=self._auth_code,
            client_secret=self._client_secret,
        )

        return token