[issue15916] change doctest DocTestSuite not to raise ValueError if no docstrings

[R. David Murray]

R. David Murray added the comment:

This looks good, however we also need a documentation change indicating the new behavior, including a '.. versionchanged:: 3.5' tag, and an entry in whatsnew/3.5 in the 'other changes' section.

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<http://bugs.python.org/issue15916>
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