Update django-anymail to 8.6

[pyup-bot]

This PR updates django-anymail[mailgun] from 8.5 to 8.6.

Changelog

8.6

--------

*2022-05-15*

This is an extended support release. Anymail v8.6 will receive security updates
and fixes for any breaking ESP API changes through at least May, 2023.

Fixes
~~~~~

* **Mailgun and SendGrid inbound:** Work around a Django limitation that
drops attachments with certain filenames. The missing attachments
are now simply omitted from the resulting inbound message. (In earlier
releases, they would cause a MultiValueDictKeyError in Anymail's
inbound webhook.)

Anymail documentation now recommends using Mailgun's and SendGrid's "raw MIME"
inbound options, which avoid the problem and preserve all attachments.

See `Mailgun inbound <https://anymail.readthedocs.io/en/stable/esps/mailgun/#mailgun-inbound>`__
and `SendGrid inbound <https://anymail.readthedocs.io/en/stable/esps/sendgrid/#sendgrid-inbound>`__
for details. (Thanks to `erikdrums`_ for reporting and helping investigate the problem.)

Other
~~~~~

* **Mailgun:** Document Mailgun's incorrect handling of display names containing
both non-ASCII characters and punctuation. (Thanks to `Flexonze`_ for spotting and
reporting the issue, and to Mailgun's `b0d0nne11`_ for investigating.)

* **Mandrill:** Document Mandrill's incorrect handling of non-ASCII attachment filenames.
(Thanks to `Thorbenl`_ for reporting the issue and following up with MailChimp.)

* Documentation (for all releases) is now hosted at anymail.dev (moved from anymail.info).

Deprecations
~~~~~~~~~~~~

*  This will be the last Anymail release to support Django 2.0--2.2 and Python 3.5.

If these deprecations affect you and you cannot upgrade, set your requirements to
`django-anymail~=8.6` (a "compatible release" specifier, equivalent to `>=8.6,==8.*`).

Links

• PyPI: https://pypi.org/project/django-anymail
• Changelog: https://pyup.io/changelogs/django-anymail/
• Repo: https://github.com/anymail/django-anymail