Django-post-office

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* Added a new `LOCK_FILE_NAME` which lets you change post office's lock file name. Thanks mogost!
* Fixes a bug where `email_queued` signal is not sent in certain cases. Thanks diesieben07!
* Fixes an issue where attachment admin page would not render with large number of emails. Thanks petrprikryl!
* Fixes a crash when email instances are made with context, but without a template. Thanks pacahon!
* Other miscellaneous fixes and house keeping tasks by mogost!

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* Added `BATCH_DELIVERY_TIMEOUT` that specifies the maximum time allowed for each batch to be delivered. Defaults to 180 seconds. Thanks selwin!

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* Optimized a queryset in `get_queued()` that doesn't use indexes in Postgres. Thanks marsha97!
* Removed `date_hierarchy` option which causes admin to load slowly on DBs with a large number of emails. Thanks selwin!
* Optimized `cleanup_expired_mails()` so that deletes emails in smaller batches. Thanks marsha97!

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* Changed JSON columns to use Django's `JSONField` and drop `jsonfield` dependency. Thanks jrief!
* Fixed saving HTML emails that have `quoted_printable`. Thanks gabn88!
* Fixes an issue where emails are rendered without context in Django's admin interface. Thanks zagl!
* This version no longer supports Django 3.1.

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* Fixed an issue where emails may not be rendered with context. Thanks zagl!
* Fixed a few packaging issues. Thanks zagl and adamchainz!
* `send_messages()` now mimics Django's SMTP Backend return value. Thanks JiriKr!

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* Improvement to attachment handling in admin interface. Thanks petrprikryl!
* Fixed a bug where HTML body is not displayed in admin interface. Thanks robbieadi!
* Explicitly specify `default_auto_field` to supress migration warnings. Thanks CirXe0N!
* Fixed a bug where `email.template` is not saved in certain cases. Thanks franciscobmacedo!