Drop support for Python 3.8

Drop support for Python 3.8

Python 3.8 went "end of life" in October 2024 and Fedora 42 dropped
this version already, so the "python" CI job is currently failing.
Thus it's time to drop support for this Python version in QEMU, too.

While we're at it, also look for "python3.13" in the configure script.

Message-ID: <20250425120710.879518-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit d64db833d6e3cbe9ea5f36342480f920f3675cea)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>