BUG: fix PyArray_IsPythonScalar to detect bytes objects on Python 3

This PR should fix `PyArray_IsPythonScalar` to return True for unibyte strings on Python 3.

As you can see [here](http://nbviewer.ipython.org/gist/immerrr/ab4a50a665844c2293aa), unibyte strings don't pass `PyString_Check` on Python 3 and only `PyBytes_Check` will work there. On 2.6/2.7 `PyBytes_Check` is an alias for `PyString_Check` (or vice versa, I don't remember), so it should work too.