Note that if we pass bytes to bind(), getsockname() will return str anyway:

>>> s = socket.socket(socket.AF_UNIX)
>>> s.bind(b'hello')
>>> s.getsockname()
'hello'

That said, it seems more consistent to me to return str also in case of abstract namespace. 
As per:
http://blog.eduardofleury.com/archives/2007/09/13
...one is supposed to set only the first byte to null, and the rest of the path is supposed to be a 'plain' string.