Enable a `latest` symlink on the ftp index

Hello all 👋
I recently talked with a friend of mine (@sdelquin) and he has a specific use-case for the ftp directory on his Python seminars and usually the students are requested to download "the latest Python version" from the command-line, from which he developed a little ad-hoc script to get such function:

PYTHON_LATEST=`curl -s https://www.python.org/downloads/ | gunzip -c | grep latest | perl -nle 'print $1 if /(\d+\.\d+\.\d+)/'`
PYTHON_DOWNLOAD=https://www.python.org/ftp/python/$PYTHON_LATEST/Python-$PYTHON_LATEST.tgz

This use-case could be improved if we have a latest symlink at https://www.python.org/ftp/python/ always pointing to the latest stable release.

I'm not certain how to implement this, because I'm not aware of how the ftp index is generated, but I have the feeling it shouldn't be so complicated, considering that during the release process, the release managers can select a checkbox to say "this is the latest release".

In my experience this is nothing new, and in some other indexes it's common to find a latest symlink, like this example

Just for the record, I already talked about this with @hugovk and @ewdurbin but I'm opening an issue so the discussion can stay in one place :)

首先跟踪 FTP 索引的生成方式，以及 release 流程如何记录最新的稳定版 release；描述没有指定具体的文件或测试。完成的标准是 Python FTP 索引公开一个指向最新稳定版 release 的 latest 符号链接，并且在 release 之后仍保持正确。