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'