Python config cleanup

Don't require Python at configure/build unless building pyzfs.
Move ZFS_AC_PYTHON_MODULE to always-pyzfs.m4 where it is used.
Make test syntax more consistent.

Sponsored by: iXsystems, Inc.
Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #8895