Fix joining of unicode and byte strings in python3

This changes the response from string to a byte string in python3, however bring it in line with _multiplexed_socket_stream_helper.

```
/bin/bash -c "echo '☺'; echo '☺'"
```
before:
```
"b'\\xe2\\x98\\xba\\n'b'\\xe2\\x98\\xba\\n'"
```
after:
```
b'\xe2\x98\xba\n\xe2\x98\xba\n'
```