Testing works properly with newer tox and virtualenv too.

--- python-dateutil-2.9.0.post0/tox.ini.orig
+++ python-dateutil-2.9.0.post0/tox.ini
@@ -17,8 +17,8 @@
 skip_missing_interpreters = true
 isolated_build = true
 requires =
-    tox<4
-    virtualenv<20.22.0
+    tox
+    virtualenv
 
 [testenv:.package]
 # no additional dependencies besides PEP 517 for building the package