Bump pygments from 2.5.2 to 2.7.4 in /docs (#9)

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>

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

docs/requirements.txt

@@ -15,7 +15,7 @@ more-itertools==8.2.0
 packaging==20.1
 pluggy==0.13.1
 py==1.8.1
-Pygments==2.5.2
+Pygments==2.7.4
 pyparsing==2.4.6
 pytest==5.3.5
 pytz==2019.3