Wait until generator is paused before running 'method'.

def _wait(self, generator, method, timeout=None, *args, **kwargs):
        """Wait until generator is paused before running 'method'."""
        if self.debug:
            print("waiting for %s to pause" % generator)

        original_timeout = timeout
        while timeout is None or timeout > 0:
            last_time = time.time()
            if self._lock.acquire(False):  # timeout param was added in 3.2
                try:
                    if self.can_resume():
                        return method(generator, *args, **kwargs)
                    elif self.has_terminated():
                        raise RuntimeError("%s has already terminated" % generator)
                finally:
                    self._lock.release()

            if timeout is not None:
                timeout -= time.time() - last_time

        msg = "%s did not pause after %ss" % (generator, original_timeout)
        if self.debug:
            print(msg)
        raise WaitTimeoutError(msg)