Request to add % formatting to ``newbytes`` (backport of PEP 461)

[edschofield]

future.types.newbytes could be extended to implement PEP 461 (__mod__ operator for bytes and bytearray formatting), which has been accepted for inclusion in Python 3.5.

This would restore % formatting like Py2's str.

Requested by Buck Goleman for the MySQLdb port to Py2/3 on the python-porting mailing list.