test: fix Django 5.1 tox env to not install 5.2

Author: browniebroke

tox.ini

@@ -30,7 +30,7 @@ deps =
     django22: Django>=2.2,<3.0
     django32: Django>=3.2,<4.0
     django42: Django>=4.2,<5.0
-    django51: Django>=5.1,<6.0
+    django51: Django>=5.1,<5.2
     djangotip: git+https://github.com/django/django.git
     pytest
     unittest-mixins==1.6