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 <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Eli Schwartz <[email protected]>
Closes #8851 

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