Just today, someone posted the result of dis.dis('somebytes') and did not notice the error because dis blithely disassembles bytes as bytecodes, even in 3.x. (The person actually dissed a 2.x string).

>>> from dis import dis
>>> dis(b'cat')
          0 DUP_TOPX        29793

It is a natural thing to do, so I hope this is put in 3.2. 

Since the undocumented 'disassemble_string' now disassembles bytes, I think it should be renamed '_disassemble_bytes' instead of '_disassemble_string'. This would accord with the general effort to remove 2.x fossils from 3.x.

Aside from that, it looks ready, from a reading review, to apply and test: doc addition, added tests, new function and else case, and rename.