Bump django-anymail[postmark] from 8.6 to 9.1

Bumps [django-anymail[postmark]](https://github.com/anymail/django-anymail) from 8.6 to 9.1.
- [Release notes](https://github.com/anymail/django-anymail/releases)
- [Changelog](https://github.com/anymail/django-anymail/blob/main/CHANGELOG.rst)
- [Commits](anymail/django-anymail@v8.6...v9.1)

---
updated-dependencies:
- dependency-name: django-anymail[postmark]
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/production.txt

@@ -1,7 +1,7 @@
 django==3.2.16
 django-allauth==0.51.0
 django-analytical==3.1.0
-django-anymail[postmark]==8.6
+django-anymail[postmark]==9.1
 django-autoslug==1.9.8
 django-constance[redis]==2.9.1
 django-crispy-forms==1.14.0