GH-128520: Subclass abc.ABC in pathlib._abc (#128745)

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.

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -31,29 +31,11 @@ def needs_windows(fn):
 #
 
 
-class JoinablePathTest(unittest.TestCase):
-    cls = JoinablePath
-
-    def test_magic_methods(self):
-        P = self.cls
-        self.assertFalse(hasattr(P, '__fspath__'))
-        self.assertFalse(hasattr(P, '__bytes__'))
-        self.assertIs(P.__reduce__, object.__reduce__)
-        self.assertIs(P.__repr__, object.__repr__)
-        self.assertIs(P.__hash__, object.__hash__)
-        self.assertIs(P.__eq__, object.__eq__)
-        self.assertIs(P.__lt__, object.__lt__)
-        self.assertIs(P.__le__, object.__le__)
-        self.assertIs(P.__gt__, object.__gt__)
-        self.assertIs(P.__ge__, object.__ge__)
-
-    def test_parser(self):
-        self.assertIs(self.cls.parser, posixpath)
-
-
 class DummyJoinablePath(JoinablePath):
     __slots__ = ('_segments',)
 
+    parser = posixpath
+
     def __init__(self, *segments):
         self._segments = segments
 
@@ -77,7 +59,7 @@ def with_segments(self, *pathsegments):
         return type(self)(*pathsegments)
 
 
-class DummyJoinablePathTest(unittest.TestCase):
+class JoinablePathTest(unittest.TestCase):
     cls = DummyJoinablePath
 
     # Use a base path that's unrelated to any real filesystem path.
@@ -94,6 +76,10 @@ def setUp(self):
         self.sep = self.parser.sep
         self.altsep = self.parser.altsep
 
+    def test_is_joinable(self):
+        p = self.cls(self.base)
+        self.assertIsInstance(p, JoinablePath)
+
     def test_parser(self):
         self.assertIsInstance(self.cls.parser, _PathParser)
 
@@ -878,6 +864,7 @@ class DummyReadablePath(ReadablePath, DummyJoinablePath):
 
     _files = {}
     _directories = {}
+    parser = posixpath
 
     def __init__(self, *segments):
         super().__init__(*segments)
@@ -909,6 +896,9 @@ def iterdir(self):
         else:
             raise FileNotFoundError(errno.ENOENT, "File not found", path)
 
+    def readlink(self):
+        raise NotImplementedError
+
 
 class DummyWritablePath(WritablePath, DummyJoinablePath):
     __slots__ = ()
@@ -942,8 +932,11 @@ def mkdir(self, mode=0o777, parents=False, exist_ok=False):
             self.parent.mkdir(parents=True, exist_ok=True)
             self.mkdir(mode, parents=False, exist_ok=exist_ok)
 
+    def symlink_to(self, target, target_is_directory=False):
+        raise NotImplementedError
 
-class DummyReadablePathTest(DummyJoinablePathTest):
+
+class ReadablePathTest(JoinablePathTest):
     """Tests for ReadablePathTest methods that use stat(), open() and iterdir()."""
 
     cls = DummyReadablePath
@@ -1010,6 +1003,10 @@ def assertEqualNormCase(self, path_a, path_b):
         normcase = self.parser.normcase
         self.assertEqual(normcase(path_a), normcase(path_b))
 
+    def test_is_readable(self):
+        p = self.cls(self.base)
+        self.assertIsInstance(p, ReadablePath)
+
     def test_exists(self):
         P = self.cls
         p = P(self.base)
@@ -1378,15 +1375,19 @@ def test_is_symlink(self):
             self.assertIs((P / 'linkA\x00').is_file(), False)
 
 
-class DummyWritablePathTest(DummyJoinablePathTest):
+class WritablePathTest(JoinablePathTest):
     cls = DummyWritablePath
 
+    def test_is_writable(self):
+        p = self.cls(self.base)
+        self.assertIsInstance(p, WritablePath)
+
 
 class DummyRWPath(DummyWritablePath, DummyReadablePath):
     __slots__ = ()
 
 
-class DummyRWPathTest(DummyWritablePathTest, DummyReadablePathTest):
+class RWPathTest(WritablePathTest, ReadablePathTest):
     cls = DummyRWPath
     can_symlink = False
 
@@ -1598,9 +1599,9 @@ def test_copy_into_empty_name(self):
         self.assertRaises(ValueError, source.copy_into, target_dir)
 
 
-class DummyReadablePathWalkTest(unittest.TestCase):
+class ReadablePathWalkTest(unittest.TestCase):
     cls = DummyReadablePath
-    base = DummyReadablePathTest.base
+    base = ReadablePathTest.base
     can_symlink = False
 
     def setUp(self):