Purge 'from typing import *' from zerver/.

This is a partial implementation of #636.

zerver/lib/parallel.py

@@ -1,6 +1,6 @@
 from __future__ import absolute_import
 from __future__ import print_function
-from typing import *
+from typing import Any, Generator, Iterable, Tuple
 
 import os
 import pty