Given an uninitialized IncrementalNewlineDecoder:
uninitialized = io.IncrementalNewlineDecoder.__new__(io.IncrementalNewlineDecoder)

each of the following calls would raise a SystemError ('null argument to
internal routine'):
uninitialized.getstate()
uninitialized.setstate((b'foo', 0))
uninitialized.reset()

In contrast, the following call would raise a ValueError
('IncrementalNewlineDecoder.__init__ not called'):
uninitialized.decode(b'bar')

ISTM that getstate(), setstate(), and reset() should have the same behavior as
decode(). (Though i think that including the actual type name in the error
message would be better, as it could be a subclass of IncrementalNewlineDecoder).