Update django-anymail to 15.0

[pyup-bot]

This PR updates django-anymail[mailgun] from 14.0 to 15.0.

Changelog

15.0

-----

*2026-04-18*

Breaking changes
~~~~~~~~~~~~~~~~

* Require **Django 5.0 or later** and Python 3.10 or later.
(Thanks to `terencehonles`_.)

Features
~~~~~~~~

* **Resend:** Add support for inbound email. (Thanks to `btimby`_.)

Fixes
~~~~~

* **Inbound:** Convert carriage return and line feed characters in attachment
filenames to spaces for ESPs that parse them from invalid headers (e.g.,
Postmark). (Thanks to `BHSPitMonkey`_ for reporting the issue and providing
an example.)

Links

• PyPI: https://pypi.org/project/django-anymail
• Changelog: https://data.safetycli.com/changelogs/django-anymail/