Declare support for Python 3.13

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.4.0
-envlist = py{39,310,311,312}
+envlist = py{39,310,311,312,313}
 isolated_build = True
 
 [testenv]
@@ -17,7 +17,7 @@ passenv =
     EPUBCHECK_PATH
     TERM
 description =
-    py{39,310,311,312}: Run unit tests against {envname}.
+    py{39,310,311,312,313}: Run unit tests against {envname}.
 extras =
     test
 setenv =