tox: add python 3.13 to default tests

With the release of Python 3.13, updating the tox configuration to
have this version of the interpreter always tested on.

Signed-off-by: James Knight <[email protected]>

Author: jdknight

tox.ini

@@ -4,7 +4,7 @@ envlist =
     pylint
     ruff
     # unit tests
-    py{37,38,39,310,311,312}
+    py{37,38,39,310,311,312,313}
     pypy3
 
 [testenv]