Add python 3.13 testing and support

As per 2026.2 cycle testing runtime, python3.13 is
mandatory python version to test
- https://governance.openstack.org/tc/reference/runtimes/2026.1.html

Adding its testing and support in tempst.

Change-Id: Ifd1d2f8f8693e6269be98be9556acc53c8e52f4c
Signed-off-by: Ghanshyam Maan <gmaan@ghanshyammann.com>
diff --git a/doc/source/supported_version.rst b/doc/source/supported_version.rst
index 789cbac..2cca456 100644
--- a/doc/source/supported_version.rst
+++ b/doc/source/supported_version.rst
@@ -36,3 +36,4 @@
 * Python 3.10
 * Python 3.11
 * Python 3.12
+* Python 3.13