GH-110109: Fix misleading pathlib._abc.PurePathBase repr (#113376)

`PurePathBase.__repr__()` produces a string like `MyPath('/foo')`. This
repr is incorrect/misleading when a subclass's `__init__()` method is
customized, which I expect to be the very common.

This commit moves the `__repr__()` method to `PurePath`, leaving
`PurePathBase` with the default `object` repr.

No user-facing changes because the `pathlib._abc` module remains private.

Lib/pathlib/_abc.py

@@ -282,9 +282,6 @@ class PurePathBase:
         slashes."""
         return str(self).replace(self.pathmod.sep, '/')
 
-    def __repr__(self):
-        return "{}({!r})".format(self.__class__.__name__, self.as_posix())
-
     @property
     def drive(self):
         """The drive prefix (letter or UNC path), if any."""