Django 6.0 support (#307)

* Remove upper bound on Django version.

This allows it to be installed alongside Django 6 and later.

See https://iscinumpy.dev/post/bound-version-constraints/

* Run tests on Django 6.0 / Python 3.12

pyproject.toml

@@ -53,7 +53,7 @@ include = [
 
 [tool.poetry.dependencies]
 python = ">=3.9,<4"
-django = ">=4.2, <6"
+django = ">=4.2"
 django-picklefield = "^3.1"
 
 blessed = { version = "^1.19.1", optional = true }