Use hashlib if running under Python 2.5

Possible change: instead of doing

    try:
        # Python 2.5 has moved to using hashlib for hashing functions
        import hashlib
        ... use hashlib
    except ImportError:
        ... use old libraries

do

    try:
        # Python 2.5 has moved to using hashlib for hashing functions
        import hashlib
    except ImportError:
        # Fallback
        ... use old libraries ...
    else:
        ... use hashlib ...

Also, this test lacks explicit tests, but I think the functionality should be covered by other existing tests.

I'm working on checking this in.

(In [6318]) Fixed #3604 -- django.contrib.auth password checking now uses hashlib if it's available. Thanks, Rob Hudson