bpo-13691: Fixes pydoc help (or help('help')) to show the doc for help

[CuriousLearner]

This is an attempt to fix issue http://bugs.python.org/issue13691

This is another approach which we could take to resolve the issue around the use of help.

Otherwise we could define a protocol such as introducing __help__ magic method as Nick suggested on the bug.

Please let me know if this needs changes :)

[the-knights-who-say-ni]

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[CuriousLearner]

CLA is signed and now visible in my account.

cc @ncoghlan

[ncoghlan]

Sorry for taking so long to review this @CuriousLearner.

I'm afraid I really don't like the notion of making pydoc natively aware of _sitebuiltins._Helper - that's two distinct cases of coupling a general purpose module shared across Python implementations to interpreter implementation details that aren't guaranteed to exist outside of CPython.

[ncoghlan]

Added the sprint label, as this PR was submitted at the PyCon Pune 2017 core development sprint.