When reading a Windows text file one byte at a time, \r\n get split into
two function calls, and the user receives two \n.

The following test fails (put it somewhere in test_io.py, inside
TextIOWrapperTest for example)

    def testReadOneByOne(self):
        txt = io.TextIOWrapper(io.BytesIO(b"AA\r\nBB"))
        reads = ""
        while True:
            c = txt.read(1)
            if not c:
                break
            reads += c
        self.assertEquals(reads, "AA\nBB")
        # AssertionError: 'AA\n\nBB' != 'AA\nBB'

Note that replacing read(1) by read(2) gives the correct result.

This problem is why test_netrc fails on Windows. It may also be the root
cause for issue 1142 (when \r\n position is just a multiple of the
_CHUNK_SIZE).
It also possible that the correction to this problem will have good
effects on test_mailbox, which uses tell() and seek() intensively.