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/google/get-google-credentials

@@ -26,8 +26,7 @@ CLIENT_SECRET_FILE = 'client_secret.json'
 APPLICATION_NAME = 'Zulip Calendar Bot'
 HOME_DIR = os.path.expanduser('~')
 
-def get_credentials():
-    # type: () -> client.Credentials
+def get_credentials() -> client.Credentials:
     """Gets valid user credentials from storage.
 
     If nothing has been stored, or if the stored credentials are invalid,