Add *preserve_metadata* keyword-only argument to `pathlib.Path.copy()`,
defaulting to false. When set to true, we copy timestamps, permissions,
extended attributes and flags where available, like `shutil.copystat()`.
The argument has no effect on Windows, where metadata is always copied.

In the pathlib ABCs we copy the file permissions with `PathBase.chmod()`
where supported. In the future we might want to support a more generic
public interface for copying metadata between different types of `PathBase`
object, but it would be premature here.