Update sphinx to 1.5.4

[pyup-bot]

There's a new version of sphinx available.
You are currently using 1.5.3. I have updated it to 1.5.4

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

Changelog

1.5.4

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

Features added

• 3470: Make genindex support all kinds of letters, not only Latin ones

Bugs fixed

• 3445: setting 'inputenc' key to \\usepackage[utf8x]{inputenc} leads
  to failed PDF build
• EPUB file has duplicated nav.xhtml link in content.opf
  except first time build
• 3488: objects.inv has broken when release or version contain
  return code
• 2073, 3443, 3490: gettext builder that writes pot files unless the content
  are same without creation date. Thanks to Yoshiki Shibukawa.
• 3487: intersphinx: failed to refer options
• 3496: latex longtable's last column may be much wider than its contents
• 3507: wrong quotes in latex output for productionlist directive
• 3533: Moving from Sphinx 1.3.1 to 1.5.3 breaks LaTeX compilation of links
  rendered as code
• 2665, 2607: Link names in C++ docfields, and make it possible for other domains.
• 3542: C++, fix parsing error of non-type template argument with template.
• 3065, 3520: python domain fails to recognize nested class
• 3575: Problems with pdflatex in a Turkish document built with sphinx has
  reappeared (refs 2997, 2397)
• 3577: Fix intersphinx debug tool
• A LaTeX command such as \\large inserted in the title items of
  :confval:latex_documents causes failed PDF build (refs 3551, 3567)

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

Happy merging! 🤖