timezone: Fix return type in get_timezone.

While it's true `datetime` is implicit via `pytz`, it makes sense
that mypy should now complain about the semantics of calling our
return type `pytz.datetime.tzinfo`, when such a type doesn't
actually exist.

zerver/lib/timezone.py

@@ -2,9 +2,10 @@
 from typing import List
 
 import pytz
+import datetime
 
 def get_all_timezones() -> List[str]:
     return sorted(pytz.all_timezones)
 
-def get_timezone(tz: str) -> pytz.datetime.tzinfo:
+def get_timezone(tz: str) -> datetime.tzinfo:
     return pytz.timezone(tz)