Bump pygments from 2.5.2 to 2.7.4 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.5.2 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.5.2...2.7.4)

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

docs/Pipfile

@@ -38,7 +38,7 @@ zipp = "==3.1.0"
 Babel = "==2.8.0"
 Jinja2 = "==2.11.1"
 MarkupSafe = "==1.1.1"
-Pygments = "==2.5.2"
+Pygments = "==2.7.4"
 Sphinx = "==2.4.3"
 
 [requires]