kingpin library deprecated

[gliptak]

https://github.com/alecthomas/kingpin

Current status. Kingpin is largely feature stable. There hasn't been a need to add new features in a while, but there are some bugs that should be fixed.

Why? I no longer use Kingpin personally (I now use [kong](https://github.com/alecthomas/kong)). Rather than leave the project in a limbo of people filing issues and wondering why they're not being worked on, I believe this notice will more clearly set expectations.

https://github.com/alecthomas/kong