Bump pygments from 2.12.0 to 2.15.0 in /docs (#131)

Bumps [pygments](https://github.com/pygments/pygments) from 2.12.0 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.12.0...2.15.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -36,7 +36,7 @@ packaging==21.3
     #   sphinx
 pydata-sphinx-theme==0.15.4
     # via sphinx-book-theme
-pygments==2.12.0
+pygments==2.15.0
     # via
     #   accessible-pygments
     #   pydata-sphinx-theme