Summary: Exception when a timezone is configured for Django and last_run_at is ambiguous.

Include a brief description of the problem here, and fill out the version info below.

• Celery Version: 4.3.0
• Celery-Beat Version: 1.5.0

Exact steps to reproduce the issue:

1. Have a timezone configured for Django and have a periodic task run during the hour before a daylight savings time change so last_run_at is during that time.
2. After the change django-celery-beat will fail with a traceback here.

Detailed information

pytz.exceptions.AmbiguousTimeError: 2019-11-03 01:00:00.336812