Is there any popular forum software (installable, not hosted), that has LDAP integration for users?
Ubuntu server, Python would be nice but not required, authentication only for now, although profiles would be nice at a later point
Have you had a look at phpBB3? You can read about their default auth settings here: http://news.softpedia.com/news/Communication-settings-in-phpBB3-65420.shtml
They offer LDAP.
Consider our AspNetForum as well, it's supports Windows-integrated authentication and integrates tightly with Active Directory http://www.jitbit.com/asp-net-forum/
It's an ASP.NET-based forum though, requires a Windows-server, not Ubuntu/Python.
PS. Please do not consider it as self-promotion since we offer a free version which is not time-limited.
Actually older version of Invision Power Board (IPB) has a mod to do that. It's the IPB 1.3. Available at IBForen > http://www.ibforen.de/forum/index.php?showtopic=13559
Don't worry, IPB 1.3 is perfectly legal to use for free (no need of license), as long as you don't receive the software through illegal means. IBForen also have the security patch and feature updates to make it relevant to modern forum software uses.