- the "ascii encoding by default" approach doesn't work in Py3K,
because a string and an ascii encoded string are of course
different things, and we'd like render() by default to return a
string.   So go the other way, use FEB in all cases,
 add a new flag bytestring_passthrough which goes back to StringIO,
to support that one guy who wanted to force a bytestring through
in an expression.