Update python version in setup.cfg

Due to requirements repo dropping the constraints for
python3.9 [1], Tempest needs to drop python 3.9
support [2]. Because Tempest master cannot support
py3.9 anymore, it needs to be dropped from tempest
plugins also.

Also, add python 3.13 which is tested as mandatory
in Gazpacho/2026.1 cycle.

[1] https://review.opendev.org/c/openstack/requirements/+/957587
[2] https://review.opendev.org/c/openstack/tempest/+/966101

Change-Id: If1c4fd7f97b7f5617f681da39eafce62ec153ab3
Signed-off-by: Brian Haley <haleyb.dev@gmail.com>
1 file changed