Bump pygments from 2.7.4 to 2.20.0 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.7.4 to 2.20.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.7.4...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]

docs/rtd-requirements.txt

@@ -58,7 +58,7 @@ Products.ZopeVersionControl==1.1.3
 Products.contentmigration==2.1.10
 Products.statusmessages==4.0
 Products.validation==2.0.1
-Pygments==2.7.4
+Pygments==2.20.0
 Record==2.13.0
 RestrictedPython==3.6.0
 Sphinx==1.1.3