Fetch access token from callback request.

def get_access_token(self, request, callback=None):
        """Fetch access token from callback request."""
        callback = request.build_absolute_uri(callback or request.path)
        if not self.check_application_state(request):
            logger.error('Application state check failed.')
            return None
        if 'code' in request.GET:
            args = {
                'client_id': self.consumer_key,
                'redirect_uri': callback,
                'client_secret': self.consumer_secret,
                'code': request.GET['code'],
                'grant_type': 'authorization_code',
            }
        else:
            logger.error('No code returned by the provider')
            return None
        try:
            response = self.request('post', self.access_token_url, data=args)
            response.raise_for_status()
        except RequestException as e:
            logger.error('Unable to fetch access token: {0}'.format(e))
            return None
        else:
            return response.text