Bump pygments from 2.1.3 to 2.15.0 in /docs (#283)

Bumps [pygments](https://github.com/pygments/pygments) from 2.1.3 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.1.3...2.15.0)

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

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docs/requirements.txt

@@ -6,7 +6,7 @@ imagesize==0.7.1
 Jinja2>=2.10.1
 MarkupSafe==0.23
 packaging==16.8
-Pygments==2.1.3
+Pygments==2.15.0
 pyparsing==2.1.10
 pytz==2016.10
 requests>=2.20.0