Fix several new errors caught by mypy 0.501.

Clear out a bunch of easy to review errors, so we can focus on the more
complicated ones.

bin/zulip-send

@@ -122,7 +122,7 @@ def main(argv=None):
 
     if not do_send_message(client, message_data):
         return 1
-
+    return 0
 
 if __name__ == '__main__':
     sys.exit(main())

integrations/hg/zulip-changegroup.py

@@ -87,7 +87,7 @@ def format_commit_lines(web_url, repo, base, tip):
     return "\n".join(summary for summary in commit_summaries)
 
 def send_zulip(email, api_key, site, stream, subject, content):
-    # type: (str, str, str, str, str, Text) -> str
+    # type: (str, str, str, str, str, Text) -> None
     """
     Send a message to Zulip using the provided credentials, which should be for
     a bot in most cases.