Bump pygments from 2.1.3 to 2.7.4 in /doc

Bumps [pygments](https://github.com/pygments/pygments) from 2.1.3 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.1.3...2.7.4)

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

Author: dependabot[bot]

doc/requirements.txt

@@ -12,7 +12,7 @@ livereload==2.4.1
 MarkupSafe==0.23
 pathtools==0.1.2
 port-for==0.3.1
-Pygments==2.1.3
+Pygments==2.7.4
 pytz==2016.1
 PyYAML==5.1
 recommonmark==0.4.0