[Feature Request] Special behavior for Temporal built-in prefixes

[cretz]

Describe the solution you'd like

See temporalio/features#576

[josh-berry]

Note: double-underscores have some special meaning in Python for method and property names on a class. Specifically, Python munges __foo -> _ClassName__foo (or something like that, I don't exactly remember).

We don't think this is relevant, because we're only reserving well-known/already-established names, and we think __temporal_ is specific enough that it's unlikely to collide with any class name.