Make library work with python 3

[mattnibs]

When installing for python 3 I get this error:

ImportError: /usr/local/lib/python3.5/dist-packages/libuuid/_uuid.cpython-35m-x86_64-linux-gnu.so: undefin
ed symbol: PyString_FromStringAndSize

This is because PyString_FromStringAndSize doesn't exist for python 3. Switch to using PyBytes_FromStringAndSize.

[brandond]

@dln are you still maintaining this? If not, would you mind handing it off?