ValueError: illegal newline value on 0x85 (NEL)

In attempting to harmonize the path pie implementation with more modern constructs, I'm seeking also to utilize the newline argument to open instead of explicitly substituting characters.

When I do, however, I find that some tests start to fail when NEL is passed as a newline character:

$ py -c "open('foo', 'w', newline='\x85')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: illegal newline value: …

At least in some contexts, Python recognizes NEL as a line separator:

>>> 'a\x85b\x1ec'.splitlines()
['a', 'b', 'c']

See also #66428 for some background.

According to the wikipedia article, a conforming application should honor a number of characters as newlines:

>>> newlines = '\x0a', '\x0b', '\x0c', '\x0d', '\x0d\x0a', '\u0085', '\u2028', '\u2029'
>>> for candidate in newlines:
...   try: x=open('foo', 'w', newline=candidate)
...   except ValueError: print('failed on', repr(candidate))
... 
failed on '\x0b'
failed on '\x0c'
failed on '\x85'
failed on '\u2028'
failed on '\u2029'

Empieza con el reproducer de open(..., newline=...) indicado y compáralo con str.splitlines() para los caracteres de nueva línea enumerados. Sigue el tratamiento de texto de I/O de CPython para la validación de newline y, después, identifica o añade las pruebas relevantes. Se considera terminado cuando los valores de newline previstos tienen un comportamiento definido y probado, sin introducir regresiones en la traducción existente de newline.