workshops/views.py: Specify sort key explicitly in match()

Avoid:

    Traceback:
      File "/.../django/core/handlers/base.py" in get_response
        111. response = wrapped_callback(request, *callback_args, **callback_kwargs)
      File "/home/wking/src/swc/amy/workshops/views.py" in match
        262. persons.sort()
    Exception Type: TypeError at /workshops/match
    Exception Value: unorderable types: Person() < Person()

In Python 3.4.