tox.ini: Add py313 and django51 to the testing

tox.ini

@@ -3,7 +3,8 @@ envlist = py3{8,9,10}-django32
         py3{8,9,10}-django41
         py3{8,9,10,11,12}-django42
         py3{10,11,12}-django50
-        py3{10,11,12}-djdev
+        py3{10,11,12,13}-django51
+        py3{10,11,12,13}-djdev
         docs
         lint
 
@@ -19,6 +20,7 @@ deps =
     django-41: Django>=4.1,<4.2
     django-42: Django>=4.2,<4.3
     django-50: Django>=5.0,<5.1
+    django-51: Django>=5.1,<5.2
     djdev: https://github.com/django/django/archive/master.tar.gz
 commands =
   {envpython} --version
@@ -32,7 +34,8 @@ setenv =
     DJANGO_TEST_PROCESSES=1
 
 basepython =
-    py311: python3.12
+    py313: python3.13
+    py312: python3.12
     py311: python3.11
     py310: python3.10
     py39: python3.9
@@ -77,3 +80,4 @@ python =
   3.10: py310
   3.11: py311
   3.12: py312
+  3.13: py313