Fix #17967 - Fix related to regression on Windows.

os.path.join(*self.dirs) produces an invalid path on windows.
ftp paths are always forward-slash seperated like this. /pub/dir.
diff --git a/Lib/urllib.py b/Lib/urllib.py
index 09a054b..244cb05 100644
--- a/Lib/urllib.py
+++ b/Lib/urllib.py
@@ -873,7 +873,8 @@
         self.ftp = ftplib.FTP()
         self.ftp.connect(self.host, self.port, self.timeout)
         self.ftp.login(self.user, self.passwd)
-        self.ftp.cwd(os.path.join(*self.dirs))
+        _target = '/'.join(self.dirs)
+        self.ftp.cwd(_target)
 
     def retrfile(self, file, type):
         import ftplib