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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 fetch_token(self, **kwargs):
 """Completes the Authorization Flow and obtains an access token. This is the final step in the OAuth 2.0 Authorization Flow. This is called after the user consents. This method calls :meth:`requests_oauthlib.OAuth2Session.fetch_token` and specifies the client configuration's token URI (usually Google's token server). Args: kwargs: Arguments passed through to :meth:`requests_oauthlib.OAuth2Session.fetch_token`. At least one of ``code`` or ``authorization_response`` must be specified. Returns: Mapping[str, str]: The obtained tokens. Typically, you will not use return value of this function and instead and use :meth:`credentials` to obtain a :class:`~google.auth.credentials.Credentials` instance. """

        kwargs.setdefault('client_secret', self.client_config['client_secret'])
        return self.oauth2session.fetch_token(
            self.client_config['token_uri'], **kwargs)