Update sphinx to 1.5.1

[pyup-bot]

There's a new version of Sphinx available.
You are currently using 1.5. I have updated it to 1.5.1

These links might come in handy: PyPI | Changelog | Homepage

Changelog

1.5.1

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

Features added

• 3214: Allow to suppress "unknown mimetype" warnings from epub builder using
  :confval:suppress_warnings.

Bugs fixed

• 3195: Can not build in parallel
• 3198: AttributeError is raised when toctree has 'self'
• 3211: Remove untranslated sphinx locale catalogs (it was covered by
  untranslated it_IT)
• 3212: HTML Builders crashes with docutils-0.13
• 3207: more latex problems with references inside parsed-literal directive
  (\DUrole)
• 3205: sphinx.util.requests crashes with old pyOpenSSL (< 0.14)
• 3220: KeyError when having a duplicate citation
• 3200: LaTeX: xref inside desc_name not allowed
• 3228: build_sphinx command crashes when missing dependency
• 2469: Ignore updates of catalog files for gettext builder. Thanks to
  Hiroshi Ohkubo.
• 3183: Randomized jump box order in generated index page.

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #25