bpo-38811: Check for presence of os.link method in pathlib. (GH-17170)

Fix also the Path.symplink() method implementation for the case when
symlinks are not supported.
diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index d70fde0..5142ff6 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -418,7 +418,12 @@
 
     unlink = os.unlink
 
-    link_to = os.link
+    if hasattr(os, "link"):
+        link_to = os.link
+    else:
+        @staticmethod
+        def link_to(self, target):
+            raise NotImplementedError("os.link() not available on this system")
 
     rmdir = os.rmdir
 
@@ -430,6 +435,7 @@
         if supports_symlinks:
             symlink = os.symlink
         else:
+            @staticmethod
             def symlink(a, b, target_is_directory):
                 raise NotImplementedError("symlink() not available on this system")
     else: