Flask-mailman

- Add configuration key `MAIL_SEND_OPTIONS` to support setting `mail_options` for `smtplib.SMTP.send_mail`
(e.g. `SMTPUTF8`) ([61](https://github.com/waynerv/flask-mailman/pull/61)).
- Pre-encodes FQDN str with punycode to improve compatibility ([66](https://github.com/waynerv/flask-mailman/pull/66)).
- Migrates as many as possible test cases from Django mail module ([64](https://github.com/waynerv/flask-mailman/pull/64)).
- Improve way of populating smtp key/cert to avoid TypeError in py>=12 ([68](https://github.com/waynerv/flask-mailman/pull/68)).

- Drop Python 3.6 support.
- Fix compatibility issue with Python 3.10
([31](https://github.com/waynerv/flask-mailman/pull/31)).
- Fix the log file generation issue to ensure that the log filename is random
([30](https://github.com/waynerv/flask-mailman/pull/30)).
- Fix compatibility issue with Python 3.12
([56](https://github.com/waynerv/flask-mailman/issues/56)).
- Support passing `from_email` in tuple format to `send_mail()` function
([35](https://github.com/waynerv/flask-mailman/issues/35)).

Added

- Add support for custom email backend.

Added

- Tox and GitHub workflows to run test, staging and release automatically.

Added

- Add support for Flask 2.0

Removed

- Drop support for Python 3.5(due to flask upgrade).

Changed

- Set `None` as the default value for the `from_email` and `recipient_list` of `send_mail()` function.