Bump pygments from 2.2.0 to 2.7.4 in /mkdocs (#225)

Bumps [pygments](https://github.com/pygments/pygments) from 2.2.0 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.2.0...2.7.4)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

mkdocs/requirements.txt

@@ -9,7 +9,7 @@ MarkupSafe==1.0
 mkdocs==0.16.3
 mkdocs-material==1.6.2
 packaging==16.8
-Pygments==2.2.0
+Pygments==2.7.4
 pymdown-extensions==3.2
 pyparsing==2.2.0
 pyyaml>=4.2b1