python3: Use "y#" instead of "s#" for binary data in PyArg_ParseTuple

python3: Use "y#" instead of "s#" for binary data in PyArg_ParseTuple

The "s#" format code for PyArg_ParseTupleAndKeywords and Py_BuildValue
converts a char* and size to/from Python str (with utf-8 encoding under
Python 3).
In some cases, we want bytes (str on Python 2, bytes on 3) instead. The
code for this is "y#" in Python 3, but that is not available in 2.

Introduce a PYARG_BYTES_LEN macro that expands to "s#" or "y#", and use
that in:
- credentials.get_ntlm_response (for input and output)
- ndr_unpack argument in PIDL generated code

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>