Add Django 5.2 support

Obtained from:

https://gitlab.com/mailman/django-mailman3/-/commit/465c1ffc77556bb8a80a678f53a40f16b9766cc6

--- pyproject.toml.orig	2026-05-11 11:32:13 UTC
+++ pyproject.toml
@@ -19,7 +19,7 @@ dependencies = [
 dependencies = [
     "django-allauth[socialaccount,openid]>=0.63",
     "django-gravatar2 >= 1.0.6",
-    "django>=4.2,<5.1",
+    "django>=4.2,<5.3",
     "mailmanclient>=3.3.3",
 ]
 requires-python = ">=3.9"