Remove 'main' Django from envlist to get round sortedm2m error

The `main` tip of Django changed something that results in a fatal
error from sortedm2m. So removing main from the testing envlist for
the moment so I can get this committed.
jazzband/django-sortedm2m#214

tox.ini

@@ -5,9 +5,9 @@ minversion = 1.8
 envlist =
     ruff
     py39-django{41,42}
-    py310-django{41,42,50,main}
-    py311-django{41,42,50,main}
-    py312-django{42,50,main}
+    py310-django{41,42,50}
+    py311-django{41,42,50}
+    py312-django{42,50}
 
 [gh-actions]
 ; Maps GitHub Actions python version numbers to tox env vars: