Speed up `pathlib.Path.iterdir()` by using `os.scandir()`

[barneygale]

We should be able to call os.scandir() from pathlib.Path.iterdir() and construct results based on the os.DirEntry.path string.

Currently we call os.listdir() and _make_child_relpath(), which returns a fully parsed/normalized string; particularly, it sets _str, _drv, _root and _tail_cached.

It's probably not worth the expense of setting _drv, _root and _tail_cached - they're only useful when paths are subsequently deconstructed with PurePath methods, which isn't particularly common. It is worth setting _str, and happily os.DirEntry.path provides a string that's very nearly normalized to pathlib's standards.

Also discussed here: https://discuss.python.org/t/is-there-a-pathlib-equivalent-of-os-scandir/46626/21

Linked PRs

• GH-117727: Speed up pathlib.Path.iterdir() by using os.scandir() #117728