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>