Bump pygments from 2.8.0 to 2.8.1 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.8.0 to 2.8.1.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/2.8.1/CHANGES)
- [Commits](pygments/pygments@2.8.0...2.8.1)

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

Author: dependabot-preview[bot]

docs/requirements.txt

@@ -9,7 +9,7 @@ sphinx-hoverxref==0.5b1
 recommonmark==0.7.1
 m2r==0.2.1
 tomlkit==0.7.0
-pygments==2.8.0
+pygments==2.8.1
 
 # Dependencies of our project:
 typing-extensions==3.7.4.3