Bump pygments from 2.1.3 to 2.7.4 in /requirements (carltongibson#1353)

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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/maintainer.txt

@@ -11,7 +11,7 @@ MarkupSafe==0.23
 pathtools==0.1.2
 pbr==1.7.0
 pkginfo==1.2.1
-Pygments==2.1.3
+Pygments==2.7.4
 pytz==2016.6.1
 PyYAML==5.4
 requests==2.20.0