Add traceback.TracebackException.__traceback__

[ericsnowcurrently]

Feature or enhancement

The traceback object passed to traceback.TracebackException is converted to StackSummary object. That is exposed as the stack attribute of the new TracebackException object.

I'd like to also expose that as __traceback__. Why? To allow TracebackException to work better as a drop-in replacement proxy for an exception object. __traceback__ is the only standard exception attribute that is not found on TracebackException.

This does imply that we may also want StackSummary/FrameSummary to likewise work as a drop-in replacement for types.TracebackType. For some prior discussion about that, see #74764 (comment).

(FWIW, I didn't find any discussion on why __traceback__ wasn't added when all the other exception attributes were. I'm guessing it was an oversight, falling outside the TracebackException use cases people were thinking about at the time.)

[ericsnowcurrently]

Also see #111921 (comment).

[ericsnowcurrently]

TracebackException isn't meant to represent an exception. Instead, it's purpose is to facilitate formatting the exception for "error display". Thus, there isn't much point to adding __traceback__.