DatabaseScheduler._get_unique_timezone_names returns multiple of the same results

[justmobilize]

While testing the version 2.8.1 to see if it's safe to use (base on #894), I found that with the new code for optimizing loading of schedules, the method DatabaseScheduler._get_unique_timezone_names was added, but without fields, distinct uses all of them. This should be written as:

    def _get_unique_timezone_names(self):
        """Get a list of all unique timezone names used in CrontabSchedule"""
        return CrontabSchedule.objects.values_list(
            'timezone', flat=True
        ).order_by('timezone').distinct('timezone')

NOTE: order_by needed for the distinct to work correctly.

Without the change, that method returns multiple of the same timezones.

[auvipy]

you are welcome to come with a patch and appropriate tests for the change