GH-73991: Add pathlib.Path.copy_into() and move_into() (#123314)

These two methods accept an *existing* directory path, onto which we join
the source path's base name to form the final target path.

A possible alternative implementation is to check for directories in
`copy()` and `move()` and adjust the target path, which is done in several
`shutil` functions. This behaviour is helpful in a shell context, but
less so in a stored program that explicitly specifies destinations. For
example, a user that calls `Path('foo.py').copy('bar.py')` might not
imagine that `bar.py/foo.py` would be created, but under the alternative
implementation this will happen if `bar.py` is an existing directory.

Lib/test/test_pathlib/test_pathlib.py

@@ -861,6 +861,14 @@ class PathTest(test_pathlib_abc.DummyPathTest, PurePathTest):
     def test_move_dangling_symlink_other_fs(self):
         self.test_move_dangling_symlink()
 
+    @patch_replace
+    def test_move_into_other_os(self):
+        self.test_move_into()
+
+    @patch_replace
+    def test_move_into_empty_name_other_os(self):
+        self.test_move_into_empty_name()
+
     def test_resolve_nonexist_relative_issue38671(self):
         p = self.cls('non', 'exist')