pygpo: use correct method flags

pygpo: use correct method flags

The METH_KEYWORDS argument must always be combined with METH_VARARGS.

In Python up to 3.7 this was checked at runtime, and as we had no callers to
get_unix_path() in Python we never noticed. In Python 3.8 it is checked at
import time, and everyone notices even if they aren't directly using GPOs.

Found and reported by Val Kulkov.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14209
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>