GH-76846, GH-85281: Call __new__() and __init__() on pathlib subclasses - #102789

This PR fixes an issue where __new__() and __init__() were not called on subclasses of pathlib.PurePath and Path in some circumstances.

Specifically, the _from_parsed_parts() constructor -- which is used when iterating directories, parents, and in with_name() and friends -- has been altered as follows:

diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index 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 to not normalize paths. Instead, paths are normalized on-demand. This also speeds up path construction, p.joinpath(q), and p / q.

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