GH-104898: Revert pathlib os.PathLike registration change. (GH-105073)

Subclassing `os.PathLike` rather than using `register()` makes
initialisation slower, due to the additional `__isinstance__` work.

This partially reverts commit bd1b622.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Lib/pathlib.py

@@ -235,7 +235,7 @@ def __repr__(self):
         return "<{}.parents>".format(type(self._path).__name__)
 
 
-class PurePath(os.PathLike):
+class PurePath:
     """Base class for manipulating paths without I/O.
 
     PurePath represents a filesystem path and offers operations which
@@ -715,6 +715,10 @@ def match(self, path_pattern, *, case_sensitive=None):
                 return False
         return True
 
+# Subclassing os.PathLike makes isinstance() checks slower,
+# which in turn makes Path construction slower. Register instead!
+os.PathLike.register(PurePath)
+
 
 class PurePosixPath(PurePath):
     """PurePath subclass for non-Windows systems.