Allow to return str instead bytes from generate_latest()

[stalkerg]

Because converting to bytes anyway happened at the end of function, have sense to return str instead bytes or have options to avoid extra transformation. (many API expected str)

[csmarchbanks]

I think the ship has sailed on this, any uses of generate_latest today will be using bytes and changing the return type would be a significant breaking change.

[stalkerg]

I think we can create a new function generate_latest_b() and return bytes, and original function will just call new function and convert to string. Seems like the current behavior comes from Python2 era.