Sympa project sources is managed with Subversion versionning system. The hosting service is sourcesup, a project hosting service managed by RENATER for the benefit of the French academic community, to promote cooperative developments.
You can download Sympa's development version at any time using a Subversion client. Note that such downloaded versions of Sympa might be uncomplete or untested versions ; tagged versions are not guaranteed until they get advertised. It's a good idea to ask Sympa authors first.
This version is based on the latest stable release ; no new feature is added, only bug fixes. This version should therefore become more and more stable with time.
Here is a sample session for getting latest development version of Sympa :
svn export https://subversion.renater.fr/sympa/trunk sympa-latest
However it's probably not idea to install an untagged version of Sympa, because it might include uncomplete code or untested new features.