Bump django-mailman3 from 1.3.12 to 1.3.15 in /web

Bumps [django-mailman3](https://gitlab.com/mailman/django-mailman3) from 1.3.12 to 1.3.15.
- [Release notes](https://gitlab.com/mailman/django-mailman3/tags)
- [Commits](https://gitlab.com/mailman/django-mailman3/compare/v1.3.12...v1.3.15)

---
updated-dependencies:
- dependency-name: django-mailman3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

web/requirements.txt

@@ -1,4 +1,4 @@
 mailmanclient==3.3.5
 postorius==1.3.10
 hyperkitty==1.3.9
-django-mailman3==1.3.12
+django-mailman3==1.3.15