Fix python3 / python2 issue in tests

If I run tests using `python3 runtests.py`, I get a failure because my
python binary is python2.

Refs #24007

    collin@anakin:~/django1.8/tests$ PYTHONPATH=.. python3 runtests.py model_regress.test_pickle
    Testing against Django installed in '/home/collin/django1.8/django'
    Creating test database for alias 'default'...
    Creating test database for alias 'other'...
    .Traceback (most recent call last):
      File "/home/collin/django1.8/tests/tmpeqwmbhyl.py", line 9, in <module>
        article = pickle.loads(data)
      File "/usr/lib/python2.7/pickle.py", line 1382, in loads
        return Unpickler(file).load()
      File "/usr/lib/python2.7/pickle.py", line 858, in load
        dispatch[key](self)
      File "/usr/lib/python2.7/pickle.py", line 886, in load_proto
        raise ValueError, "unsupported pickle protocol: %d" % proto
    ValueError: unsupported pickle protocol: 3
    F.
    ======================================================================
    FAIL: test_unpickling_when_appregistrynotready (model_regress.test_pickle.ModelPickleTestCase)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/collin/django1.8/tests/model_regress/test_pickle.py", line 85, in test_unpickling_when_appregistrynotready
        result = subprocess.check_output(['python', script.name])
    subprocess.CalledProcessError: Command '['python', '/home/collin/django1.8/tests/tmpeqwmbhyl.py']' returned non-zero exit status 1

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "/home/collin/django1.8/tests/model_regress/test_pickle.py", line 87, in test_unpickling_when_appregistrynotready
        self.fail("Unable to reload model pickled data")
    AssertionError: Unable to reload model pickled data

    ----------------------------------------------------------------------
    Ran 3 tests in 0.062s

    FAILED (failures=1)
    Destroying test database for alias 'default'...
    Destroying test database for alias 'other'...