Tests: Update to Django 6.0; fall back to 5.2 on Python <3.12 (#887)

Author: intgr

pyproject.toml

@@ -46,7 +46,8 @@ dev = [
   "pytest==9.0.2",
   "pytest-mypy-plugins==3.2.0",
   "djangorestframework==3.16.1",
-  "django==5.2.*",
+  "django==5.2.*; python_version < '3.12'",
+  "django==6.0.*; python_version >= '3.12'",
   "types-pytz==2025.2.0.20251108",
   "types-requests==2.32.4.20250913",
   "types-urllib3==1.26.25.14",