Update sphinx to 1.5.3

[pyup-bot]

There's a new version of sphinx available.
You are currently using 1.5.2. I have updated it to 1.5.3

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

Changelog

1.5.3

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

Features added

• Support requests-2.0.0 (experimental) (refs: 3367)
• (latex) PDF page margin dimensions may be customized (refs: 3387)
• literalinclude directive allows combination of :pyobject: and
  :lines: options (refs: 3416)
• 3400: make-mode doesn't use subprocess on building docs

Bugs fixed

• 3370: the caption of code-block is not picked up for translation
• LaTeX: :confval:release is not escaped (refs: 3362)
• 3364: sphinx-quickstart prompts overflow on Console with 80 chars width
• since 1.5, PDF's TOC and bookmarks lack an entry for general Index
  (refs: 3383)
• 3392: 'releasename' in :confval:latex_elements is not working
• 3356: Page layout for Japanese 'manual' docclass has a shorter text area
• 3394: When 'pointsize' is not 10pt, Japanese 'manual' document
  gets wrong PDF page dimensions
• 3399: quickstart: conf.py was not overwritten by template
• 3366: option directive does not allow punctuations
• 3410: return code in :confval:release breaks html search
• 3427: autodoc: memory addresses are not stripped on Windows
• 3428: xetex build tests fail due to fontspec v2.6 defining \strong
• 3349: Result of IndexBuilder.load() is broken
• 3450: &nbsp is appeared in EPUB docs
• 3418: Search button is misaligned in nature and pyramid theme
• 3421: Could not translate a caption of tables

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

Happy merging! 🤖