Django-cron

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- Fixed error in file locking backend with Python 3

- Fixed `'NoneType' object has no attribute 'utcoffset'` error

- Updated unit tests and demo for Django 2.0 compatibility

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- Added support for Django 1.10

- Minimum Django version required is 1.8

- Use parser.add_argument() instead of optparse.make_option() in runcrons command

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- Model import error fix for Django 1.9.X

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- Added ability to check how many time left until next run.

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- Remove max_length from CronJobLog.message field.

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- Added DJANGO_CRON_DELETE_LOGS_OLDER_THAN setting to allow automated log clearing.