TorNet: replace os.symlink with Path.symlink_to

I seem to have glanced over Path.symlink_to() in pathlib documentation,
or forgotten to change this line of code, one of the two

lib/chutney/TorNet.py

@@ -2162,7 +2162,7 @@ class Network(object):
                 pass
             else:
                 raise
-        os.symlink(newnodesdir, nodeslink)
+        newnodesdir.symlink_to(nodeslink)
 
     def _checkConfig(self):
         for n in self._nodes: