What system?  test_idle passes on Windows.  Unfortunately, none of the *nix or Mac buildbots run tkinter and hence no tests that require a live tk window, and this one does.  Cheryl, what do you see on your linux?

Byte 0xc3 in position 1540 is the first byte of the utf-8 encoding of the 'ö' in Löwis on line 27 of CREDITS.txt.  So on your system, readline must be reading *and decoding* more of the file.

I am puzzled that the test fails now if it passed with the 3.6.4 release, as it and the textfile have not been touched since last summer.
The files are the same on all 3 python versions.

In any case, I will add encoding='utf-8'to open.  I will also use subtest to identify failing cases.  I will need you to verify the fix on your system.