Bump django-mailman3 from 1.3.8 to 1.3.9 in /web (#588)

Bumps [django-mailman3](https://gitlab.com/mailman/django-mailman3) from 1.3.8 to 1.3.9.
- [Release notes](https://gitlab.com/mailman/django-mailman3/tags)
- [Commits](https://gitlab.com/mailman/django-mailman3/compare/v1.3.8...1.3.9)

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

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

web/requirements.txt

@@ -1,4 +1,4 @@
 mailmanclient==3.3.5
 postorius==1.3.8
 hyperkitty==1.3.7
-django-mailman3==1.3.8
+django-mailman3==1.3.9