Used main branch for Django REST framework edge testing

Removed testing for Python 3.9 on DRF main as support got removed
already in DRF main branch.

Author: sliverc

tox.ini

@@ -1,8 +1,8 @@
 [tox]
 envlist =
-    py{39,310,311,312}-django42-drf{315,316,master},
-    py{310,311,312}-django{51,52}-drf{315,316,master},
-    py{313}-django{51,52}-drf{316,master},
+    py39-django42-drf{315,316},
+    py{310,311,312}-django{42,51,52}-drf{315,316,main},
+    py{313}-django{51,52}-drf{316,main},
     black,
     docs,
     lint
@@ -14,7 +14,7 @@ deps =
     django52: Django>=5.2,<5.3
     drf315: djangorestframework>=3.15,<3.16
     drf316: djangorestframework>=3.16,<3.17
-    drfmaster: https://github.com/encode/django-rest-framework/archive/master.zip
+    drfmain: https://github.com/encode/django-rest-framework/archive/main.zip
     -rrequirements/requirements-testing.txt
     -rrequirements/requirements-optionals.txt