TOX: Add a Python 3.13 environment

Author: effigies

tox.ini

@@ -16,7 +16,7 @@ envlist =
   # x64-only range
   py312-{full,pre}-x64
   # Special environment for numpy 2.0-dev testing
-  py312-dev-x64
+  py313-dev-x64
   install
   doctest
   style
@@ -31,6 +31,7 @@ python =
   3.10: py310
   3.11: py311
   3.12: py312
+  3.13: py313
 
 [gh-actions:env]
 DEPENDS =