Bump pygments from 2.1.1 to 2.7.4 in /doc

Bumps [pygments](https://github.com/pygments/pygments) from 2.1.1 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.1.1...2.7.4)

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

doc/requirements.txt

@@ -10,7 +10,7 @@ Jinja2==2.8
 livereload==2.4.1
 MarkupSafe==0.23
 pathtools==0.1.2
-Pygments==2.1.1
+Pygments==2.7.4
 pytz==2015.7
 PyYAML==3.11
 recommonmark==0.4.0