Bump pygments from 2.1.3 to 2.7.4 in /docs (#225)

Bumps [pygments](https://github.com/pygments/pygments) from 2.1.3 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.1.3...2.7.4)

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

docs/requirements.txt

@@ -8,7 +8,7 @@ docutils==0.12
 imagesize==0.7.1
 Jinja2==2.11.3
 MarkupSafe==0.23
-Pygments==2.1.3
+Pygments==2.7.4
 pytz==2016.4
 six==1.10.0
 snowballstemmer==1.2.1