Bump pygments from 2.7.4 to 2.15.0 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.7.4 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.7.4...2.15.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -10,7 +10,7 @@ mkdocs==1.2.3
 mkdocs-material==7.0.3
 mkdocs-material-extensions==1.0.1
 nltk==3.6.6
-Pygments==2.7.4
+Pygments==2.15.0
 pymdown-extensions==8.0.1
 PyYAML==5.4.1
 regex==2020.9.27