zerver/lib: Use python 3 syntax for typing.

Split by tabbott from a larger commit; this covers a batch of files
with no open PRs touching them.

zerver/lib/camo.py

@@ -7,8 +7,7 @@ from typing import Text
 
 # Encodes the provided URL using the same algorithm used by the camo
 # caching https image proxy
-def get_camo_url(url):
-    # type: (Text) -> Text
+def get_camo_url(url: Text) -> Text:
     # Only encode the url if Camo is enabled
     if settings.CAMO_URI == '':
         return url