Add a 'type: ignore' in zulip_tools.py.

Subprocess.CalledProcessError's __init__ takes 4 arguments out
of which 2 are optional.  It is being passed 2 arguments in
zulip_tools.py. However, python 3 stubs are incorrect and only
1 argument has been marked as default.
See https://github.com/python/typeshed/pull/329.
Add '# type: ignore' to that line so that mypy check passes in
python 3 mode.

zulip_tools.py

@@ -75,5 +75,5 @@ def run(args):
     process = subprocess.Popen(args)
     rc = process.wait()
     if rc:
-        raise subprocess.CalledProcessError(rc, args)
+        raise subprocess.CalledProcessError(rc, args) # type: ignore # https://github.com/python/typeshed/pull/329
     return 0