Fix fish_config URL generation for python3

In Python3, the following code (line 864) produces a bytes object, not a string:

`authkey = binascii.b2a_hex(os.urandom(16))`

Resulting in fish_config opening a URL like this:

`http://localhost:8000/b'86583ea6eebe3a1c60ca0648f64d9782'/`

This patch properly decodes the bytes object when running Python3, fixing the issue.