So just what are the specs for this old laptop?
Depending its capabilities, instead of focusing on 'lightweight' I'm going to suggest you instead opt for a distro that's more 'functional'. A solid, tried-and-true distro like Debian running a window manager like Fluxbox will give you a stable OS with a much more complete feature set while requiring a lot of system resources. If you're only looking to run a basic email client and a Python console, a fully functional networking/protocol stack running in the background is going to be a benefit. If you'll be in a situation where you need your laptop to also be doing anything like running a Samba or NFS server, you'll find it easier than using on of those smaller, lightweight distros.