Skip monkey-patching for South with Django 1.7

This fixes the following error when South tries to access
`management._commands`, which has been dropped in Django 1.7:

    …/pytest_django/pytest_django/fixtures.py:41: in _django_db_setup
        _handle_south()
    …/pytest_django/pytest_django/fixtures.py:128: in _handle_south
        patch_for_test_db_setup()
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

        def patch_for_test_db_setup():
            # Load the commands cache
            management.get_commands()
            # Repoint to the correct version of syncdb
            if hasattr(settings, "SOUTH_TESTS_MIGRATE") and not settings.SOUTH_TESTS_MIGRATE:
                # point at the core syncdb command when creating tests
                # tests should always be up to date with the most recent model structure
                management._commands['syncdb'] = 'django.core'
            else:
    >           management._commands['syncdb'] = MigrateAndSyncCommand()
    E           AttributeError: 'module' object has no attribute '_commands'

    …/site-packages/south/management/commands/__init__.py:34: AttributeError


I wonder if we should output some warning here, because South is not meant to be used with Django 1.7?!

The patch/diff looks huge, but is mostly whitespace / indentation.