Bump pygments from 2.15.0 to 2.20.0

Bumps [pygments](https://github.com/pygments/pygments) from 2.15.0 to 2.20.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.15.0...2.20.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-version: 2.20.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -2,7 +2,7 @@ pyscf
 sphinx
 sphinxcontrib-bibtex
 nbsphinx
-Pygments==2.15.0
+Pygments==2.20.0
 # MarkupSafe 2.1.1 breaks Jinja2 2.10.1
 MarkupSafe==2.0.1
 pydata-sphinx-theme