Avoid impossible constraints

tox.ini

@@ -14,5 +14,5 @@ deps =
     dj32: Django>=3.2,<4.0
     dj42: Django>=4.2,<5.0
     dj50: Django>=5.0,<5.1
-    dj51: Django>=5.1,<5.1
+    dj51: Django>=5.1,<5.2
     djmain: https://github.com/django/django/archive/main.tar.gz