pytest has several instances where plugins set their own attributes on
objects they receive in hooks, like nodes and config. Since plugins are
detached from these object's definition by design, this causes a problem
for type checking because these attributes are not defined and mypy
complains.
Fix this by giving these objects a "store" which can be used by plugins
in a type-safe manner.
Currently this mechanism is private. We can consider exposing it at a
later point.
