Lib: Add pathlib test for missing symlink support

This adds a test case to test_pathlib.py to test the NotImplementedError
raised when symlink support is not present on the system. Also add the
missing @staticmethod decorator to the function definition in pathlib.py
for that error path.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: tohojo

Lib/pathlib.py

@@ -435,6 +435,7 @@ def link_to(self, target):
         if supports_symlinks:
             symlink = os.symlink
         else:
+            @staticmethod
             def symlink(a, b, target_is_directory):
                 raise NotImplementedError("symlink() not available on this system")
     else:

Lib/test/test_pathlib.py

@@ -2021,6 +2021,15 @@ def test_symlink_to(self):
         self.assertTrue(link.is_dir())
         self.assertTrue(list(link.iterdir()))
 
+    @unittest.skipIf(support.can_symlink(), "symlink support is present")
+    def test_symlink_to_not_implemented(self):
+        P = self.cls(BASE)
+        target = P / 'fileA'
+        # Symlinking a path target.
+        link = P / 'dirA' / 'linkAA'
+        with self.assertRaises(NotImplementedError):
+            link.symlink_to(target)
+
     def test_is_dir(self):
         P = self.cls(BASE)
         self.assertTrue((P / 'dirA').is_dir())