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.

Lib/urllib/request.py

@@ -2276,7 +2276,8 @@ def init(self):
         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