[python] In Python 3, integers are long but in GDL they are int

This is observed when testing #468, with Python 3.7.4.
```
function ARG_PASS_RETURN, arg
  return, arg
end
```
In Python 3, integers are long but in GDL they are int. So, the leading bits are ignored:
```
>>> import GDL
>>> arg = 2**45-1
>>> retval = GDL.function('arg_pass_return', arg)
% Compiled module: ARG_PASS_RETURN.
>>> print(retval)
-1
```
