Remove tested support for Django 4.1; add tests for Django 5.1 and Py…

…thon 3.13

Aligning with the Django supported versions (see
https://www.djangoproject.com/download/#supported-versions),
but leaving support for Django 3.2, so that we at least
know if and when to explicitly call out dropping support
for that LTS release in the README.

tox.ini

@@ -1,10 +1,11 @@
 [tox]
 envlist =
     py38-django{32}
-    py39-django{32,41,42}
-    py310-django{32,41,42,50}
-    py311-django{41,42,50}
-    py312-django{42,50}
+    py39-django{32,42}
+    py310-django{32,42,50,51}
+    py311-django{42,50,51}
+    py312-django{42,50,51}
+    py313-django{51}
 
 [testenv]
 set_env =
@@ -17,6 +18,6 @@ commands =
 deps =
     djangorestframework<4
     django32: Django>=3.2,<3.3
-    django41: Django>=4.1,<4.2
     django42: Django>=4.2,<4.3
     django50: Django>=5.0,<5.1
+    django51: Django>=5.1,<5.2