GH-128520: Make pathlib._abc.WritablePath a sibling of ReadablePath

[barneygale]

In the private pathlib ABCs, support write-only virtual filesystems by making WritablePath inherit directly from JoinablePath, rather than subclassing ReadablePath. There are two complications:

Problem 1: ReadablePath.open() applies to both reading and writing

Solution: 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().

Problem 2: ReadablePath.copy is secretly an object that supports the read side of copying, whereas WritablePath.copy is a subclass additionally supporting the write side

Solution: 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. Once GH-125413 is resolved, we'll be able to move the CopyReader functionality into ReadablePath.info and eliminate ReadablePath._copy_reader.

• Issue: Make pathlib ABCs usable by zipfile.Path #128520