Event object no longer has "membership" key

[rht]

First reported at zulip/python-zulip-api#612. For example, code like

matrix-python-sdk/samples/SimpleChatClient.py

Line 26 in a469d53

if event['membership'] == "join":

no longer works.

The list of keys of the event object as of now:
['unsigned', 'origin_server_ts', 'type', 'room_id', 'content', 'sender', 'event_id'].