Sphinx-testing

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- Support Sphinx-2.0.1

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- Support Sphinx-2.0 (unreleased yet)
- Fix a bug:

- 12: with_app decorator should return the value of the decorated function

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- Fix a bug:

- 9: warningiserror=True is broken with Sphinx 1.8.2

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- Drop py33 support
- Fix bug:

- 8: Tests are failing with Sphinx 1.8.2

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- Fix bug:

- 2: Fix cleanups for Sphinx 1.6

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- Fix bug:

- Fix write_docstring should refer master_doc and source_suffix