tox.ini: update testing in Django 5.1

Author: PetrDlouhy

tox.ini

@@ -29,7 +29,7 @@ deps=
     django41: Django>=4.1,<4.2
     django42: Django>=4.2,<5.0
     django50: Django>=5.0,<5.1
-    django51: git+https://github.com/django/django@main  # TODO: update when Django 5.1 is released
+    django51: Django>=5.1a1,<5.2
     coverage[toml]
     pillow
     ipdb