Python: Re-introduce Response::instance() in flask model
Python: Re-introduce Response::instance() in flask model

We don't actually need it for anything right now, but I have plans for the
future where would need it.

Although it would be nice to have it as an `API::Node`, and we could re-write
implementations so we could provide it in this instance, I'm not convinced we
can do that in general right now.

For example, if <n'th> parameter of a function has to be modeled as belonging to
a certain type, I don't see any way to specify that as an API::Node.

For me, that's ok. Until we _can_ specify things like this as API::Nodes in the
future, I would like to keep things consistent, and use `DataFlow::Node` as the
result type.

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