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'

報告された open(..., newline=...) の reproducer から始め、列挙された改行文字について str.splitlines() と比較します。newline の検証に関する CPython のテキスト I/O 処理を追跡し、その後、関連するテストを特定するか追加します。意図された newline の値に、既存の newline 変換を後退させることなく、定義済みでテストされた動作があれば完了です。