Update sphinx to 1.5.6

[pyup-bot]

There's a new version of sphinx available.
You are currently using 1.5.5. I have updated it to 1.5.6

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

Changelog

1.5.6

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

Bugs fixed

• 3614: Sphinx crashes with requests-2.5.0
• 3618: autodoc crashes with tupled arguments
• 3664: No space after the bullet in items of a latex list produced by Sphinx
• 3657: EPUB builder crashes if document startswith genindex exists
• 3588: No compact (p tag) html output in the i18n document build even when
  :confval:html_compact_lists is True.
• 3685: AttributeError when using 3rd party domains
• 3702: LaTeX writer styles figure legends with a hard-coded \small
• 3708: LaTeX writer allows irc scheme
• 3717: Stop enforcing that favicon's must be .ico
• 3731, 3732: Protect isenumclass predicate against non-class arguments
• 3320: Warning about reference target not being found for container types
• Misspelled ARCHIVEPREFIX in Makefile for latex build repertory

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

Happy merging! 🤖