[docs] [issue17279] Document which named built-in classes can be subclassed

[Terry J. Reedy]

Terry J. Reedy added the comment:

>From the current python-ideas 'range' thread:
Me: Would it be correct to say (now) that all 4 are intentional omissions? and not merely oversights?
Nick: Yes, I think so. People will have to be *real* convincing to explain a case where composition isn't a more appropriate solution.

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<http://bugs.python.org/issue17279>
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