Fix get_binary_data to use py::bytes

[lauralindzey]

Hi @msis -- this is probably only a partial fix, but I figured I'd submit a PR anyways in case it helps you work towards a more robust fix.

Caveats:

• I haven't tested this on python2, so it may need additional work to be backwards-compatible
• I haven't tested publishing a bytes object, since my application only requires subscribing to them.