python3 support

This patch set provides support to use python3. 

When building with python3, the numpy headers has the `import_array()` macro return a value and this causes a compilation error because the functions it resides it are void and shouldn't have one. To work around this we can just override the return value macro, `NUMPY_IMPORT_ARRAY_RETVAL`, by re-defining it to nothing so the return statement has no effect. This is the only change needed to the code as far as I can tell. 

Set `PYTHON_VER=3` before building and testing.