Bump pygments from 2.11.2 to 2.15.0 in /docs (#47)

Bumps [pygments](https://github.com/pygments/pygments) from 2.11.2 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.11.2...2.15.0)

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

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docs/requirements.txt

@@ -10,7 +10,7 @@ importlib-metadata==4.11.3
 jinja2==3.1.1
 markupsafe==2.1.1
 packaging==21.3
-pygments==2.11.2
+pygments==2.15.0
 pyparsing==3.0.8
 readthedocs-sphinx-search==0.1.1
 requests==2.31.0