Update sphinx to 2.1.1

[pyup-bot]

This PR updates Sphinx from 2.1.0 to 2.1.1.

Changelog

2.1.1

==============================

Dependencies
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Incompatible changes
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Deprecated
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Features added
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Bugs fixed
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* 6442: LaTeX: admonitions of :rst:dir:`note` type can get separated from
immediately preceding section title by pagebreak
* 6448: autodoc: crashed when autodocumenting classes with ``__slots__ = None``
* 6452: autosummary: crashed when generating document of properties

Testing
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Links

• PyPI: https://pypi.org/project/sphinx
• Changelog: https://pyup.io/changelogs/sphinx/
• Homepage: http://sphinx-doc.org/