_helpers.c: be smarter when converting unknown integer types to python.

For now assume that long long and unsigned long long are always the
the largest integer types, and convert unknown size integers to Python
via PyLong_FromUnsignedLongLong() or PyLong_FromLongLong(), depending
on the value's sign.

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