Python: Use only `TApiNode` in `API::Impl`
Python: Use only `TApiNode` in `API::Impl`

This ensures that changes to `API::Node` does not invalidate the cached
`module Impl`. At present, I don't expect this to have any effect (as
the `Node` class is also fairly static, though not explicitly cached),
but I can imagine us making some of the `Node` methods have
user-extensible behaviour, in which case we definitely do not want this
to result in reevaluation of `API::Impl`.

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