zerver/lib/notifications.py: Add hints for mypy.

Add type hints for mypy using isinstance in assert and if.

tools/run-mypy

@@ -42,7 +42,6 @@ exclude_py2 = []
 exclude_py3 = """
 zerver/lib/actions.py
 zerver/lib/ccache.py
-zerver/lib/notifications.py
 zerver/lib/tornado_ioloop_logging.py
 zerver/management/commands/create_stream.py
 zerver/management/commands/email-mirror.py

zerver/lib/notifications.py

@@ -53,7 +53,7 @@ def hashchange_encode(string):
     # frontend.
     # `safe` has a default value of "/", but we want those encoded, too.
     return urllib.parse.quote(
-        string.encode("utf-8"), safe="").replace(".", "%2E").replace("%", ".")
+        string.encode("utf-8"), safe=b"").replace(".", "%2E").replace("%", ".")
 
 def pm_narrow_url(participants):
     # type: (List[text_type]) -> text_type