rpcfs doesn't work with python 3

for:
`fsserve -t rpc -p 1917`
and then:
`fstree rpc://127.0.0.1:1917`
it gives this output:

```
fstree: Error - a bytes-like object is required, not 'str'
rpc://127.0.0.1:1917
```

didn't check with python 2 but this one look pretty much python 3 asks for bytes where old python 2 code sends string.
