build(deps): bump pygments from 2.6.1 to 2.7.4 in /docs

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](pygments/pygments@2.6.1...2.7.4)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -37,7 +37,7 @@ mkdocs==1.1.2
     #   mkdocs-material
 nltk==3.5
     # via lunr
-pygments==2.6.1
+pygments==2.7.4
     # via mkdocs-material
 pymdown-extensions==7.1
     # via mkdocs-material