LGTM.

But the test fails with PYTHONIOENCODING=ascii.

$ PYTHONIOENCODING=ascii ./python -m test test_readline
Run tests sequentially
0:00:00 [1/1] test_readline
test test_readline failed -- Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/test/test_readline.py", line 194, in test_nonascii
    self.assertIn(b"result " + expected + b"\r\n", output)
AssertionError: b"result '[\\xefnserted]|t\\xebxt[after]'\r\n" not found in bytearray(b'^A^B^B^B^B^B^B^B\t\tx\t\r\n[\xc3\xafnserted]|t\xc3\xab[after]\x08\x08\x08\x08\x08\x08\x08text \'t\\xeb\'\r\nline \'[\\xefnserted]|t\\xeb[after]\'\r\nindexes 11 13\r\n\x07text \'t\\xeb\'\r\nline \'[\\xefnserted]|t\\xeb[after]\'\r\nindexes 11 13\r\nsubstitution \'t\\xeb\'\r\nmatches [\'t\\xebnt\', \'t\\xebxt\']\r\nx[after]\x08\x08\x08\x08\x08\x08\x08t[after]\x08\x08\x08\x08\x08\x08\x08\r\nTraceback (most recent call last):\r\n  File "<string>", line 39, in <module>\r\nUnicodeDecodeError: \'ascii\' codec can\'t decode byte 0xc3 in position 1: ordinal not in range(128)\r\n')

This is minor problem, since buildbots rarely configured with PYTHONIOENCODING=ascii.