dectree.py uses env python3 rather than configured python

[Dr. David Alan Gilbert]

Hi Taylor,
  I tripped over dectree.py using 'env python3'; the qemu configure
script lets you specify a python with e.g.:
    --with-python=/usr/libexec/platform-python

and I think everywhere else in qemu uses the configured python.
(This host, like most standard rhel8, doesn't have a python3 binary out
of the box)

Dave

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK