Try to find the token for the provided request token key.

def _get_token(self, oauth_request, token_type='access'):
        """Try to find the token for the provided request token key."""
        token_field = oauth_request.get_parameter('oauth_token')
        token = self.data_store.lookup_token(token_type, token_field)
        if not token:
            raise OAuthError('Invalid %s token: %s' % (token_type, token_field))
        return token