Update sphinx requirement from 1.8.3 to 2.1.2

[dependabot-preview]

Updates the requirements on sphinx to permit the latest version.

Changelog

Sourced from sphinx's changelog.

Release 2.1.2 (released Jun 19, 2019)

Bugs fixed

• #6497: custom lexers fails highlighting when syntax error
• #6478, #6488: info field lists are incorrectly recognized

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()

... (truncated)

Commits

• 39563d1 Bump to 2.1.2 final
• 131c93c Remove "universal" configuration from setup.cfg
• 2709cd3 Update CHANGES for PR #6482
• 982f63e Merge pull request #6482 from ViktorHaag/object-descr-setting-class-type-map-...
• e9ee39d repair white space
• 6a05435 Simplify to ensure we're lazily setting the instance field-type-map variable
• be08561 Merge pull request #6502 from tk0miya/6497_custom_lexers_fails_highlighting
• 2119a65 Fix #6497: custom lexers fails highlighting when syntax error
• d19dd1c mypy type annotations for methods added
• 085ae2e type declaration for new internal method
• Additional commits viewable in compare view

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting @dependabot rebase.

---

Dependabot commands and options

You can trigger Dependabot actions by commenting on this PR:

• @dependabot rebase will rebase this PR
• @dependabot recreate will recreate this PR, overwriting any edits that have been made to it
• @dependabot merge will merge this PR after your CI passes on it
• @dependabot squash and merge will squash and merge this PR after your CI passes on it
• @dependabot cancel merge will cancel a previously requested merge and block automerging
• @dependabot reopen will reopen this PR if it is closed
• @dependabot ignore this [patch|minor|major] version will close this PR and stop Dependabot creating any more for this minor/major version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this dependency will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)
• @dependabot use these labels will set the current labels as the default for future PRs for this repo and language
• @dependabot use these reviewers will set the current reviewers as the default for future PRs for this repo and language
• @dependabot use these assignees will set the current assignees as the default for future PRs for this repo and language
• @dependabot use this milestone will set the current milestone as the default for future PRs for this repo and language
• @dependabot badge me will comment on this PR with code to add a "Dependabot enabled" badge to your readme

Additionally, you can set the following in your Dependabot dashboard:

• Update frequency (including time of day and day of week)
• Automerge options (never/patch/minor, and dev/runtime dependencies)
• Pull request limits (per update run and/or open at any time)
• Out-of-range updates (receive only lockfile updates, if desired)
• Security updates (receive only security updates, if desired)

Finally, you can contact us by mentioning @dependabot.

[dependabot-preview]

Superseded by #371.