_helpers.c: use "unsigned long long" in ASSIGN_PYLONG_TO_INTEGRAL().

The result of PyLong_AsUnsignedLongLong() was being assigned to an
"unsigned long".  Don't do that.

Thanks to Alexander Barton <alex@barton.de> for reporting the problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>