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
Merge "Fixed a PyPy compatibility issue in the test fixtures"