This PR fixes an issue where__new__() and__init__() were not called on subclasses ofpathlib.PurePath andPath in some circumstances.

Specifically, the_from_parsed_parts() constructor -- which is used when iterating directories, parents, and inwith_name() and friends -- has been altered as follows:

diff --git a/Lib/pathlib.py b/Lib/pathlib.pyindex 55c44f12e5..c9b45ddbfa 100644--- a/Lib/pathlib.py+++ b/Lib/pathlib.py@@ -310,7 +310,9 @@ def _from_parts(cls, args):      @classmethod     def _from_parsed_parts(cls, drv, root, parts):-        self = object.__new__(cls)+        path = cls._format_parsed_parts(drv, root, parts)+        self = cls(path)+        self._str = path or '.'         self._drv = drv         self._root = root         self._parts = parts

This change alone has an unfortunate effect: paths constructed this way are re-parsed and re-normalized even though we have the fully normalized path at hand.

To fix this, we change the main constructor tonot normalize paths. Instead, paths are normalized on-demand. This also speeds up path construction,p.joinpath(q), andp / q.

• Issue:pathlib.Path._from_parsed_parts should call cls.__new__(cls) #76846
• Issue:subclasses of pathlib.PurePosixPath never call __init__ or __new__ #85281