problematic String.serialize/deserialize behaviour for bytes in python 3

[novocaine]

If colander.String() is passed bytes data on python 3 and self.encoding is None, it does str() on the bytes object.

This is almost always undesirable on python 3 as, unlike python 2, there is no implicit encoding peformed and instead you get a 'b' and some quotes whacked in:

>>> str(b'test')
"b'test'"

Note that if the interpreter is running with -b or -bb this is a warning or an error respectively.

In my view, colander should raise an Exception in this case under python 3 as there is no sane way to make it into an actual string in the absence of an encoding.