tests: use python3 as the python executable name

Use "python3" instead of "python" as per PEP0394:
https://peps.python.org/pep-0394/



This should always be defined (in a venv, at least!), matching the
preferred python shebang of "#!/usr/bin/env python3".

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-4-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>