Reintroduce use of setup_requirements writer

It was broken since commit 3bbd42903af8, which changed chr(0) (which is '\x00') into bytes([0]).
bytes([0]) is '[0]' in Python 2.6 and 2.7 and b'\x00' in Python 3.

remove warning on normalization

indicate that normalization is happening, but don't be pushy about changing valid versions.

--HG--
branch : no-normalize-warning