MAL> The user would in both cases type 'python setup.py install' MAL> but the install command would automatically choose the MAL> right target subdir (site-packages/ or system-packages/).
Except you can't always tell if its a system package or an add-on. email for example is an add-on for Python 2.1, but a system package for Python 2.2.
assuming that the package maintainer is informed when a package is added to the standard library, that packages won't move in and out too much, and/or that most users probably don't want to down- grade to an older package version, you could of course write:
if sys.version_info >= (2, 2): pkgtype = "system-package" else: pkgtype = "site-package"
setup(... pkgtype=pkgtype ...)
in your setup.py file, once your package has been added.