rm improper assert syntax

assert only raise AssertionError when runing python without '-O', so if
we except an Assertionerror in codes, just raise it instead of using
assert syntax instead of different result with or without '-O' flag

Change-Id: I8c1bc2c6849996f0d3f7a4b791671871d79dd939

keystoneclient/middleware/auth_token.py

@@ -667,8 +667,8 @@ class AuthProtocol(object):
         try:
             token = data['access']['token']['id']
             expiry = data['access']['token']['expires']
-            assert token
-            assert expiry
+            if not (token and expiry):
+                raise AssertionError('invalid token or expire')
             datetime_expiry = timeutils.parse_isotime(expiry)
             return (token, timeutils.normalize_time(datetime_expiry))
         except (AssertionError, KeyError):