Update sphinx requirement from 1.8.3 to 2.1.1

[dependabot-preview]

Updates the requirements on sphinx to permit the latest version.

Changelog

Sourced from sphinx's changelog.

Release 2.1.1 (released Jun 10, 2019)

Incompatible changes

• #6447: autodoc: Stop to generate document for undocumented module variables

Bugs fixed

• #6442: LaTeX: admonitions of :rst:dir:note type can get separated from
  immediately preceding section title by pagebreak
• #6448: autodoc: crashed when autodocumenting classes with __slots__ = None
• #6451: autodoc: generates docs for "optional import"ed modules as variables
• #6452: autosummary: crashed when generating document of properties
• #6455: napoleon: docstrings for properties are not processed
• #6436: napoleon: "Unknown target name" error if variable name ends with
  underscore
• #6440: apidoc: missing blank lines between modules

Release 2.1.0 (released Jun 02, 2019)

Incompatible changes

• Ignore filenames without file extension given to Builder.build_specific()
  API directly
• #6230: The anchor of term in glossary directive is changed if it is consisted
  by non-ASCII characters
• #4550: html: Centering tables by default using CSS
• #6239: latex: xelatex and xeCJK are used for Chinese documents by default
• Sphinx.add_lexer() now takes a Lexer class instead of instance. An
  instance of lexers are still supported until Sphinx-3.x.

Deprecated

• sphinx.builders.latex.LaTeXBuilder.apply_transforms()
• sphinx.builders._epub_base.EpubBuilder.esc()
• sphinx.directives.Acks
• sphinx.directives.Author
• sphinx.directives.Centered
• sphinx.directives.Class
• sphinx.directives.CodeBlock
• sphinx.directives.Figure
• sphinx.directives.HList
• sphinx.directives.Highlight
• sphinx.directives.Include

... (truncated)

Commits

• afa4a77 Bump to 2.1.1 final
• 1eb8e01 Merge pull request #6471 from tk0miya/6447_revert_undoc_module_level_variables
• 8828017 Update CHANGES
• 21be789 Revert "Fix #1063: autodoc: automodule directive handles undocumented module ...
• 2289ae0 Merge pull request #6458 from tk0miya/6451_autodoc_optonal_imported_modules
• 1148f9f Merge branch '2.1.1' into 6451_autodoc_optonal_imported_modules
• c3b93c1 Update CHANGES for PR #6445
• 0b95f3f Merge pull request #6460 from tk0miya/6436_napoleon_unknown_target_name
• bc61713 Merge branch '2.1.1' into 6436_napoleon_unknown_target_name
• 783de00 Merge pull request #6457 from tk0miya/6455_napoleon_ignores_properties
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #365.