call Python scripts directly instead of calling python

This should make it easier for downstream packaging for Debian and FreeBSD to
redirect to a different version of Python by simply patching shebangs.

Github: related to #167 "switch all Python dependencies to Python 3 in
  packaging/debian"
Debian: related to #943253 "rumur: Python2 removal in sid/bullseye"
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=943253