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-version: 2.15.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/requirements.txt

@@ -16,7 +16,7 @@ astroid ==2.2.5
 nbconvert <6.0.0
 jinja2 ==2.11.3
 docutils ==0.16
-pygments==2.7.4
+pygments==2.15.0
 alabaster==0.7.12
 scipy
 numpy