Bump pygments from 2.6.1 to 2.7.4 in /backend

Bumps [pygments](https://github.com/pygments/pygments) from 2.6.1 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.6.1...2.7.4)

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

backend/requirements.txt

@@ -89,7 +89,7 @@ protobuf==3.9.1
 psutil==5.7.0
 ptyprocess==0.6.0
 pycparser==2.20
-Pygments==2.6.1
+Pygments==2.7.4
 PyNaCl==1.3.0
 pyparsing==2.4.2
 PyPDF2==1.26.0