QA: Convert nodes from items to attributes

The new objects support attributes, which are cleaner than
dictionary-style access. Item access (like a dictionary) is going away
in a couple of patches. Also, pylint is better at checking attributes
than dictionary entries.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>