> That would be incorrect for some systems. For example, FreeBSD does
> change sets of symbolic constants across system releases (mostly
> additions, but sometimes also removals). Back then, SunOS 4 and SunOS
> 5 were completely unrelated systems.
>

Well, I don't see the problem in that case.
If what's intended is to target different operating systems releases,
then sys.platform is the right choice and should be kept, since it
embeds the major version number.
The point I (and others) have been trying to make is that 99% of the
time, people using sys.platform really mean platform.system() or
uname[0], since they're only interested in the operating system, and
don't care about the release. That's true of the vast majority of such
occurrences in Lib/test, and probably true of the vast majority of the
user code base.
Furthermore, at least on Linux, the major version number doesn't mean
anything, since features are added and removed in "minor" versions,
and Linux 3.0 doesn't have anything special as far as features as
concerned (as explained by Linus, a version number is just a number).