slack-bridge: Use threading instead of multiprocessing.
This will fix the bug reported in
which is specific to macOS.
The error message is
```
AttributeError: Can't pickle local object 'SlackBridge.zulip_to_slack.<locals>._zulip_to_slack'
```
MODIFY
zulip/integrations/bridge_with_slack/run-slack-bridge
zulip/integrations/bridge_with_slack/run-slack-bridge
@@ -5,7 +5,7 @@ import sys
import os
import argparse
import traceback
-import multiprocessing as mp
+import threading
import zulip
import slack_sdk
from slack_sdk.rtm import RTMClient
@@ -123,8 +123,8 @@ if __name__ == "__main__":
try:
sb = SlackBridge(config)
- zp = mp.Process(target=sb.zulip_client.call_on_each_message, args=(sb.zulip_to_slack(),))
- sp = mp.Process(target=sb.run_slack_listener, args=())
+ zp = threading.Thread(target=sb.zulip_client.call_on_each_message, args=(sb.zulip_to_slack(),))
+ sp = threading.Thread(target=sb.run_slack_listener, args=())
print("Starting message handler on Zulip client")
zp.start()
print("Starting message handler on Slack client")

slack-bridge: Use threading instead of multiprocessing.

This will fix the bug reported in
https://chat.zulip.org/#narrow/stream/127-integrations/topic/bridge_with_slack/near/1182555,
which is specific to macOS.

The error message is
```
AttributeError: Can't pickle local object 'SlackBridge.zulip_to_slack.<locals>._zulip_to_slack'
```