bpo-24132: Add test for direct subclassing of PurePath/Path in pathlib

Add expected failing tests to pathlib that will verify the ability
of PurePath and Path to be subclassed by users. As part of this,
test all of the black-box class properties of the newly subclassed
Path-like class instances, and the instances generated by their methods.

Lib/test/test_pathlib.py

@@ -1341,6 +1341,30 @@ def test_instance_class_properties(self):
         self.assertClassProperties(p, correct_cls)
         self.assertNonIONewInstancesClassProperties(p, correct_cls)
 
+    @unittest.expectedFailure
+    def test_direct_subclassing(self):
+        P = self.cls
+        try:
+            class Derived(P):
+                pass
+        except Exception as e:
+            self.fail(f"Failed to subclass {P}: {e}")
+        else:
+            try:
+                derived = Derived('non_empty_pathsegment')
+            except Exception as e:
+                self.fail("Failed to be able to instantiate a class "
+                          f"derived from {P}: {e}")
+            else:
+                # Unlike how PurePath behaves, when instantiated, the
+                # instances of its user-created direct subclass keep
+                # their original class name (instead of becoming a
+                # flavour-named variant). Hence we use Derived here.
+                correct_cls = Derived
+                self.assertClassProperties(derived, correct_cls)
+                self.assertNonIONewInstancesClassProperties(derived,
+                                                            correct_cls)
+
     def test_different_flavours_unequal(self):
         p = pathlib.PurePosixPath('a')
         q = pathlib.PureWindowsPath('a')
@@ -2480,6 +2504,36 @@ def test_instance_class_properties(self):
             link = P(BASE, "linkA")
             self.assertClassProperties(link.readlink(), correct_cls)
 
+    @unittest.expectedFailure
+    def test_direct_subclassing(self):
+        P = self.cls
+        try:
+            class Derived(P):
+                pass
+        except Exception as e:
+            self.fail(f"Failed to subclass {P}: {e}")
+        else:
+            try:
+                derived_base = Derived(BASE)
+            except Exception as e:
+                self.fail("Failed to be able to instantiate a class "
+                          f"derived from {P}: {e}")
+            else:
+                # Much like in the original version of this method,
+                # we use Derived here because unlike Path, user-created
+                # subclasses of Path keep their names when instantiated
+                # (instead of becoming a flavour-named variant).
+                correct_cls = Derived
+                self.assertClassProperties(derived_base, correct_cls)
+                self.assertNonIONewInstancesClassProperties(derived_base,
+                                                            correct_cls)
+                self.assertIONoLinkNewInstancesClassProperties(derived_base,
+                                                              correct_cls)
+                if os_helper.can_symlink():
+                    derived_link = Derived(BASE, "linkA")
+                    self.assertClassProperties(derived_link.readlink(),
+                                               correct_cls)
+
     def test_class_getitem(self):
         self.assertIs(self.cls[str], self.cls)