GH-128520: Subclass abc.ABC in pathlib._abc

[barneygale]

Convert JoinablePath, ReadablePath and WritablePath to real ABCs derived from abc.ABC.

Make JoinablePath.parser abstract, rather than defaulting to posixpath.

Register PurePath and Path as virtual subclasses of the ABCs rather than deriving. This avoids a hit to path object instantiation performance.

No change of behaviour in the public (non-abstract) classes.

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

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 RHEL8 LTO 3.x has failed when building commit a7d41a8.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/338/builds/8082) and take a look at the build logs.
4. Check if the failure is related to this commit (a7d41a8) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/338/builds/8082

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 1054, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/test_interpreters/test_stress.py", line 47, in run
    interp = interpreters.create()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/support/interpreters/__init__.py", line 76, in create
    id = _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
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