Just reviewed the documentation for both os.sep and os.path.join().

The os.sep docs do not suggest it can be set, and the reference in the os.path.join() description is silent regarding that, so can be read as Jared did.  I don't recall this coming up before, but... there's a lot I don't remember at this point.  --sigh--

Removing the reference to os.sep from the os.path.join() description would not be confusing, IMO.  Clearly, the ambiguity can bite.

I'd be open to a patch to remove the reference to os.sep from the os.path.join() docs.  Reopening, but removing 3.6 and 3.7 since they're in security-fix-only mode.