Make call of os.getppid() conditional: it is not available on Windows.

Doc/library/multiprocessing.rst

@@ -81,6 +81,7 @@ To show the individual process IDs involved, here is an expanded example::
     def info(title):
         print title
         print 'module name:', __name__
+        if hasattr(os, 'getppid'):  # only available on Unix
             print 'parent process:', os.getppid()
         print 'process id:', os.getpid()