Upgrade Sphinx to 1.5.1

[fabaff]

Release 1.5.1

Features added

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

Bugs fixed

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

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bbangert and @robbiet480 to be potential reviewers.