Add Django 5.2 support

Obtained from:

https://gitlab.com/mailman/postorius/-/commit/0468ab0329df85b89e6b5d9f7b4d1805f47450c9

--- pyproject.toml.orig	2026-05-11 11:01:34 UTC
+++ pyproject.toml
@@ -18,7 +18,7 @@ dependencies = [
 ]
 dependencies = [
     "django-mailman3>=1.3.13",
-    "django>=4.2,<5.1",
+    "django>=4.2,<5.3",
     "mailmanclient>=3.3.3",
     "readme_renderer[md]",
 ]