[PyUP] Update readme-renderer to 20.0

[pyup-bot]

This PR updates readme-renderer from 17.2 to 20.0.

Changelog

20.0

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* Reformat docutils messages into more human-friendly output. (92)
* Fix incorrect escaping of html entities in pre tags when using markdown. (89)
* Allow width, height, alt, and align attributes on img tags. (91)
* Fix check to not report success when there is invalid RST. (90)

19.0

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* Allow details and summary tags. (77)
* Add .pytest_cache to .gitignore. (85)
* Warn about Markdown content type when checking RST. (83)
* Update pep8 tox env to use py3.6. (84)
* Add Pygments-based syntax highlighting for Markdown. (82)
* Update docs about check to talk about Markdown. (80)

18.1

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* Change Github-flavored Markdown identifier from ``gfm`` to ``GFM``.

18.0

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* Add support for GitHub-flavored Markdown. (67)
* Switch to cmarkgfm completely for rendering Markdown. (68)
* Warn about missing long description. (69)
* Make Github-Flavored Markdown the default variant (73)

17.4

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* All renderers now accept arbitrary ``kwargs`` for future-proofing.

17.3

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* Gracefully handle new exceptions from bleach.

Links

• PyPI: https://pypi.org/project/readme-renderer
• Changelog: https://pyup.io/changelogs/readme-renderer/
• Repo: https://github.com/pypa/readme_renderer

[pyup-bot]

Closing this in favor of #84