xml.sax.saxutils.XMLGenerator._write tests the argument by 
isinstance(text, str), but this is problematic in Python 3.0. 
XMLGenerator accepts encoding and the produced file is encoded by that 
encoding, i.e., the XML is a binary sequence. So IMHO, the XMLGenerator 
constructor argument should be a subclass of io.RawIOBase.

To clarify the specific problem:

- If the file object passed to XMLGenerator is opened in binary mode,
XMLGenerator raises TypeError as soon as it tries to write to it
- If the passed file object is opened in text mode, XMLGenerator writes
the prescribed encoding to the XML declaration but it actually uses the
file object's encoding when writing everything

Patch attached.  This patch doesn't actually restrict the output object
to RawIOBase (that wouldn't work well, since files opened as binary are
actually derived from BufferedIOBase).  Instead, it just assumes the
output object has a 'write' method that accepts a single bytes argument.
 Also, XMLGenerator no longer needs to check if the input is str or unicode.

Actually, that patch may not work so well either... out defaults to
sys.stdout, but that can't accept bytes.

This new patch removes the "default to stdout" behavior.

Patch for documentation.

You shouldn't remove the defaulting behaviour for `out`, but use
`sys.stdout.buffer` instead.

Bonus points if you add a test so that this kind of bug doesn't go
unnoticed again.

PS: it's ironic that the default encoding here is iso-8859-1. This piece
of code is really getting old.