GH-125413: Add private metadata methods to pathlib.Path.info (#129897)

Add the following private methods to `pathlib.Path.info`:

- `_posix_permissions()`: the POSIX file permissions (`S_IMODE(st_mode)`)
- `_file_id()`: the file ID (`(st_dev, st_ino)`)
- `_access_time_ns()`: the access time in nanoseconds (`st_atime_ns`)
- `_mod_time_ns()`: the modify time in nanoseconds (`st_mtime_ns`)
- `_bsd_flags()`: the BSD file flags (`st_flags`)
- `_xattrs()`: the file extended attributes as a list of key, value pairs,
  or an empty list if `listxattr()` or `getxattr()` fail in an ignorable 
  way.

These methods replace `LocalCopyReader.read_metadata()`, and so we can
delete the `CopyReader` and `LocalCopyReader` classes. Rather than reading
metadata via `source._copy_reader.read_metadata()`, we instead call
`source.info._posix_permissions()`, `_access_time_ns()`, etc.

Preserving metadata is only supported for local-to-local copies at the
moment. To support copying metadata between arbitrary `ReadablePath` and
`WritablePath` objects, we'd need to make the new methods public and
documented.

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Lib/pathlib/_abc.py

@@ -14,7 +14,7 @@
 from abc import ABC, abstractmethod
 from glob import _PathGlobber, _no_recurse_symlinks
 from pathlib import PurePath, Path
-from pathlib._os import magic_open, CopyReader, CopyWriter
+from pathlib._os import magic_open, CopyWriter
 
 
 def _explode_path(path):
@@ -353,8 +353,6 @@ def readlink(self):
         """
         raise NotImplementedError
 
-    _copy_reader = property(CopyReader)
-
     def copy(self, target, follow_symlinks=True, dirs_exist_ok=False,
              preserve_metadata=False):
         """