`os.walk()` no longer supports late edits to `dirnames`

[barneygale]

Prior to the fix for #89727, it was possible influence which subdirectories were visited by os.walk() even after the walk had descended into siblings of those subdirectories. Such "late" modifications no longer have any effect.

This was reported by @pochmann:

I think you changed os.walk's behavior. The recursive one supported late modifications of subdirs lists, due to being lazier. The iterative one doesn't (if I'm not mistaken... I can't test).

With topdown, you can modify a directory's subdirs, for example remove one and it won't be visited. Usually you'd do that while your walk is on that parent directory. But with the recursive one, it was possible to do it later. With the iterative one, I don't think that works anymore.

Here's a demo where I remove the second subdir after having walked onto the first:

import os

# Create demo dir with three subdirs
os.makedirs('demo/a')
os.makedirs('demo/b')
os.makedirs('demo/c')

# Walk onto "demo"
walk = os.walk('demo')
demo = next(walk)
print(demo)

# Walk onto first subdir
first = next(walk)
print(first)

# Remove the second subdir
del demo[1][1]

# Walk onto the remaining subdirs
for x in walk:
    print(x)

Output (Try it online!), note that the second subdir wasn't walked:

('demo', ['c', 'a', 'b'], [])
('demo/c', [], [])
('demo/b', [], [])

Excerpt from the recursive one:

        for dirname in dirs:
            new_path = join(top, dirname)
            yield from _walk(new_path, topdown, onerror, followlinks)

The iteration of dirs and the walking of the subdirs are intertwined. After walking a subdir, the paused iteration of dirs resumes. That allows the late modifications of dirs to have an effect.

Excerpt from the iterative one:

            for dirname in reversed(dirs):
                new_path = join(top, dirname)
                stack.append(new_path)

This eagerly puts all subdirs onto the stack, before they're getting walked, and then dirs is never used again. So modifying dirs during the subdirs walking doesn't have an effect anymore.

[jonburdo]

If we do want to revert to the old behavior, this PR should do it (or at least shows how it could be done, including a unit test): #100703

Currently, the latest commit splits the implementation for topdown=True and topdown=False into two functions, but a previous commit also shows how it could be done without splitting:
https://github.com/python/cpython/pull/100703/files/1011e199529ccdbd778226574c379403708b2e5d