arc_summary: prefer python3 version and install when there is no python

This matches the behavior of other python scripts, such as arcstat and
dbufstat, which are always installed but whose install-exec-hook actions
will simply touch up the shebang if a python interpreter was configured
*and* that interpreter is a python2 interpreter.

Fixes installation in a minimal build chroot without python available.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@freqlabs.com>
Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Closes #8851