Update github-markup requirement from ~> 3.0 to ~> 4.0

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Updates the requirements on github-markup to permit the latest version.

Changelog

Sourced from github-markup's changelog.

4.0.0 - 2021-03-31

• Drop support for Python 2 in RST rendering #1456

3.0.5 - 2020-11-12

• Add commonmarker_exts to commonmarker options #1268
• Check whether filename is set when rendering Asciidoc. #1290

3.0.4 - 2019-04-03

• Expose options in #render_s #1249
• Upgrade to Asciidoctor 2.0.x #1264

3.0.3 - 2018-12-17

• Temporarily remove support for POD6 #1248

3.0.2 - 2018-12-12

• Add support for POD6 #1173

3.0.1 - 2018-10-19

• Remove linguist-detected RMarkdown files from the Markdown renderer #1237

3.0.0 - 2018-10-18

• Allow passing options through to CommonMarker #1236
• Symlink option is now a keyword arg #1236

2.0.2 - 2018-10-15

• Don't render rmd files as Markdown #1235

2.0.1 - 2018-06-29

• Create anchor for every =item directive in POD files #1165

2.0.0 - 2018-01-31

• Remove filesystem access #1157

1.7.0 - 2018-01-30

Changed

• Updates for Linguist v6 #1139
• Update to Nokogiri ~> 1.8; drop support for Ruby 2.0 #1156

... (truncated)

Commits

• 0ed8aff Merge pull request #1456 from github/python3-rst
• 4230afe Add changelog entry
• 9899397 Update HTML fixture for python3
• 935ee3b Update bootstrap script to use pip3
• e1fb435 Remove -S flag for python3
• b95a306 Point to python3 executable for rendering RST
• 194e363 bump version to 3.0.5
• cd01f9e Merge pull request #1290 from dometto/fix1289
• 2710ab9 Check whether filename is set when rendering asciidoc
• f3531a5 Merge pull request #1274 from maloucaze/use-heredoc
• Additional commits viewable in compare view

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