future.builtins.newbytes: find and rfind should return a position

Stumbled on this while using newbytes/newstr in my application. 
find returns a position, so when we override it we should not try to convert it somehow. 
Ran tests (test_bytes.py) on py3 and py2, seems fine. Other tests seem to be broken im master, so i couldn't really check them. 

Currently based on master, I can rebase if the dev/bugfix-branch is another one.