Drop python 3.8 . Add tests for python 3.13 and django 5.1

tox.ini

@@ -1,16 +1,17 @@
 [tox]
 envlist =
-    py{38,39,310,311}-dj-4.2
-    py{310,311,312}-5.0
-    py{310,311,312}-dj-master
+    py{39,310,311}-dj-4.2
+    py{310,311,312,313}-5.0
+    py{310,311,312,313}-5.1
+    py{310,311,312,313}-dj-master
 
 [gh-actions]
 python =
-    3.8: py38
     3.9: py39
     3.10: py310
     3.11: py311
     3.12: py312
+    3.13: py313
 
 [testenv]
 usedevelop = true
@@ -22,4 +23,5 @@ setenv=
 deps =
     dj-4.2: Django>=4.2,<5
     dj-5.0: Django>=5,<5.1
+    dj-5.1: Django>=5.1,<5.2
     dj-master: https://github.com/django/django/archive/master.tar.gz