I can't think of a good solution, though. Stepping back a bit, I think
it's unfortunate that python blocks handle ":results value" differently
depending on whether the block is hooked up to a session or not. For
non-sessions, you have to use return. Using the same approach
(org-babel-python-wrapper-method) for ":session :results value", we
could then get the return value reliably, but the problem with this
approach is that any variables defined in a ":results value" block
wouldn't be defined in the session after executing the block because the
code is wrapped in a function.