py3: Make sure to specify METH_VARARGS together with METH_KEYWORDS

py3: Make sure to specify METH_VARARGS together with METH_KEYWORDS

A Python 3 bug https://bugs.python.org/issue15657 explains that one should
always use METH_VARARGS|METH_KEYWORDS when defining a function rather
than a lonely METH_KEYWORDS. We had only one definition like this in
Samba and it was the one that affects FreeIPA when running in Python 3
mode.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12905

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>