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](pygments/pygments@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

@@ -32,7 +32,7 @@ nbsphinx==0.8.0
 nest-asyncio==1.4.2
 packaging==20.4
 pandocfilters==1.4.3
-Pygments==2.7.4
+Pygments==2.15.0
 pyparsing==2.4.7
 pyrsistent==0.17.3
 python-dateutil==2.8.1