GH-73991: Add pathlib.Path.copytree() (#120718)

Add `pathlib.Path.copytree()` method, which recursively copies one
directory to another.

This differs from `shutil.copytree()` in the following respects:

1. Our method has a *follow_symlinks* argument, whereas shutil's has a
   *symlinks* argument with an inverted meaning.
2. Our method lacks something like a *copy_function* argument. It always
   uses `Path.copy()` to copy files.
3. Our method lacks something like a *ignore_dangling_symlinks* argument.
   Instead, users can filter out danging symlinks with *ignore*, or
   ignore exceptions with *on_error*
4. Our *ignore* argument is a callable that accepts a single path object,
   whereas shutil's accepts a path and a list of child filenames.
5. We add an *on_error* argument, which is a callable that accepts
   an `OSError` instance. (`Path.walk()` also accepts such a callable).

Co-authored-by: Nice Zombies <nineteendo19d0@gmail.com>

Lib/test/test_pathlib/test_pathlib.py

@@ -653,6 +653,19 @@ class PathTest(test_pathlib_abc.DummyPathTest, PurePathTest):
             self.assertIsInstance(f, io.RawIOBase)
             self.assertEqual(f.read().strip(), b"this is file A")
 
+    @unittest.skipIf(sys.platform == "win32" or sys.platform == "wasi", "directories are always readable on Windows and WASI")
+    def test_copytree_no_read_permission(self):
+        base = self.cls(self.base)
+        source = base / 'dirE'
+        target = base / 'copyE'
+        self.assertRaises(PermissionError, source.copytree, target)
+        self.assertFalse(target.exists())
+        errors = []
+        source.copytree(target, on_error=errors.append)
+        self.assertEqual(len(errors), 1)
+        self.assertIsInstance(errors[0], PermissionError)
+        self.assertFalse(target.exists())
+
     def test_resolve_nonexist_relative_issue38671(self):
         p = self.cls('non', 'exist')