auth pycredentials: incorrect PyArg_ParseTupleAndKeywords call

The challenge parameter was being treated as a string rather than as a
data blob.  This was causing intermittent seg faults. Removed the
server_timestamp parameter as it's not currently used.

Unable to produce a test case to reliably replicate the failure.
However auth_log_samlogon does flap

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
