A couple of suggestions:

- if IncrementalNewlineDecoder gets an encoding argument, it can also
instantiate the decoder itself; that way the API is a bit simpler

- to encode '\r' without the BOM, you can e.g. use an incremental
encoder and encode it twice:
>>> enc = codecs.getincrementalencoder('utf16')('strict')
>>> enc.encode('\r')
b'\xff\xfe\r\x00'
>>> enc.encode('\r')
b'\r\x00'


I think breaking the API can be ok since the original API is broken
(witness this bug). There can even be a compatibility mode where we
check whether `encoding` has an encode() method, and if it has, take it
as the decoder object rather than the encoding name.