typing: Convert function type annotations to Python 3 style.

Generated by com2ann (slightly patched to avoid also converting
assignment type annotations, which require Python 3.6), followed by
some manual whitespace adjustment, and two fixes for use-before-define
issues:

-    def set_zulip_client(self, zulipToJabberClient: ZulipToJabberBot) -> None:
+    def set_zulip_client(self, zulipToJabberClient: 'ZulipToJabberBot') -> None:

-def init_from_options(options: Any, client: Optional[str] = None) -> Client:
+def init_from_options(options: Any, client: Optional[str] = None) -> 'Client':

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zulip/integrations/zephyr/sync-public-streams

@@ -13,8 +13,7 @@ import zulip
 
 from typing import Set, Optional
 
-def fetch_public_streams():
-    # type: () -> Optional[Set[bytes]]
+def fetch_public_streams() -> Optional[Set[bytes]]:
     public_streams = set()
 
     try: