Bump pygments from 2.6.1 to 2.7.4 in /docs (#725)

Bumps [pygments](https://github.com/pygments/pygments) from 2.6.1 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.6.1...2.7.4)

Signed-off-by: dependabot[bot] <support@github.com>

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

docs/requirements.txt

@@ -14,7 +14,7 @@ mkdocs-monorepo-plugin==0.4.11
 mkdocs-redirects==1.0.1
 nltk==3.5b1
 prompt-toolkit==2.0.10
-Pygments==2.6.1
+Pygments==2.7.4
 pymdown-extensions==7.0
 python-markdown-math==0.6
 PyYAML==5.4