…blePath`In the private pathlib ABCs, support write-only virtual filesystems bymaking `WritablePath` inherit directly from `JoinablePath`, rather thansubclassing `ReadablePath`.There are two complications:- `ReadablePath.open()` applies to both reading and writing- `ReadablePath.copy` is secretly an object that supports the *read* side  of copying, whereas `WritablePath.copy` is a different kind of object  supporting the *write* sideWe untangle these as follow:- A new `pathlib._abc.magic_open()` function replaces the `open()` method,  which is dropped from the ABCs but remains in `pathlib.Path`. The  function works like `io.open()`, but additionally accepts objects with  `__open_rb__()` or `__open_wb__()` methods as appropriate for the mode.  These new dunders are made abstract methods of `ReadablePath` and  `WritablePath` respectively.  If the pathlib ABCs are made public, we  could consider blessing an "openable" protocol and supporting it in  `io.open()`, removing the need for `pathlib._abc.magic_open()`.- `ReadablePath.copy` becomes a true method, whereas `WritablePath.copy` is  deleted. A new `ReadablePath._copy_reader` property provides a  `CopyReader` object, and similarly `WritablePath._copy_writer` is a  `CopyWriter` object. OncepythonGH-125413 is resolved, we'll be able to move  the `CopyReader` functionality into `ReadablePath.info` and eliminate  `ReadablePath._copy_reader`.