Bump pygments from 2.2.0 to 2.7.4 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.2.0 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.2.0...2.7.4)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/docs_requirements.txt

@@ -1,3 +1,3 @@
 Sphinx==1.8.1
-Pygments==2.2.0
+Pygments==2.7.4
 Sphinx==1.8.1