Fixed a PyPy compatibility issue in the test fixtures

This construct didn't work under PyPy, as a result of the fact that
datetime is implemented in Python in PyPy:

A simplification of the problem is this:

import datetime

class X(datetime.datetime):
    pass

now = datetime.datetime.now()
X.isoformat(now)

This works under CPython, but fails under PyPy. This is because PyPy
creates a normal unbound method for `X.isoformat`, which checks that
`now` is an instance of `X`, which it isn't. Because datetime is
implemented in C, CPython skips those checks.

Change-Id: Id27e7a8b00156bd9688bdb36d2c38b04785c2d0c

openstack_dashboard/test/test_data/keystone_data.py

@@ -250,7 +250,7 @@ def data(TEST):
     TEST.tenant = tenant  # Your "current" tenant
 
     tomorrow = datetime_safe.datetime.now() + timedelta(days=1)
-    expiration = datetime_safe.datetime.isoformat(tomorrow)
+    expiration = tomorrow.isoformat()
 
     scoped_token_dict = {
         'access': {