Declare Python 3.13 support

Python 3.13 is part of supported runtimes for 2026.1[1] and now is
tested.

[1] https://governance.openstack.org/tc/reference/runtimes/2026.1.html

Change-Id: I6b5bf46a8d3cf1c04269a387e37225ddce1eae85
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

Author: kajinamit

setup.cfg

@@ -20,6 +20,7 @@ classifier =
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11
     Programming Language :: Python :: 3.12
+    Programming Language :: Python :: 3.13
 
 [files]
 packages =