tox.ini: Temporarily stop testing Django v6.0a1

GitHub Actions tests are [currently failing](https://github.com/django-commons/django-fsm-2/actions/workflows/test.yml) on Django's `main` branch, so temporarily stop testing Django v6.0 alpha.
* https://pypi.org/project/Django/#history
* https://github.com/django/django/tags

Author: cclauss

tox.ini

@@ -3,8 +3,8 @@ envlist =
     py{38,39,310,311}-dj42
     py{310,311,312}-dj50
     py{310,311,312}-dj51
-    py{310,311,312,313}-dj52
-    py{312,313}-djmain
+    py{310,311,312,313,314}-dj52
+    # py{312,313,314}-djmain
 
 skipsdist = True