Fetch access token from callback request.

def get_access_token(self, request, callback=None):
        "Fetch access token from callback request."
        raw_token = request.session.get(self.session_key, None)
        verifier = request.GET.get('oauth_verifier', None)
        if raw_token is not None and verifier is not None:
            data = {'oauth_verifier': verifier}
            callback = request.build_absolute_uri(callback or request.path)
            callback = force_text(callback)
            try:
                response = self.request('post', self.provider.access_token_url,
                                        token=raw_token, data=data, oauth_callback=callback)
                response.raise_for_status()
            except RequestException as e:
                logger.error('Unable to fetch access token: {0}'.format(e))
                return None
            else:
                return response.text
        return None