Fix slow_watch_mock

[jkawamoto]

This PR fixes "TypeError: 'NoneType' object is not iterable" mentioned in #1959 (comment).

This error happens because Watcher._run expects the Watch callable object returns an iterable:

python-etcd3/etcd3/watch.py

Lines 128 to 136 in e78000e

	def _run(self):
	callback_err = None
	try:
	response_iter = self._watch_stub.Watch(
	_new_request_iter(self._request_queue),
	credentials=self._credentials,
	metadata=self._metadata)
	for rs in response_iter:
	self._handle_response(rs)

but slow_watch_mock, which is used in test_watch_timeout_on_establishment returns nothing:

python-etcd3/tests/test_etcd3.py

Lines 379 to 382 in e78000e

	def slow_watch_mock(*args, **kwargs):
	time.sleep(4)
	foo_etcd.watcher._watch_stub.Watch = slow_watch_mock # noqa

This PR modifies slow_watch_mock to return an empty list to avoid the type error.

Also, it updates Dockerfile to use Python 3.9 and newer etcd.

[kragniz]

Thanks!