As we want to enforce a unique and explicit Python 3 interpreter,
we need let this script handle 'python3' too.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200130163232.10446-3-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>