Bump pygments from 2.6.1 to 2.7.4 in /src/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](https://github.com/pygments/pygments/compare/2.6.1...2.7.4)

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

src/backend/requirements.txt

@@ -20,7 +20,7 @@ django-anymail[mailgun]==7.0.0
 botocore==1.15.34
 boto3==1.12.34
 python-magic==0.4.15
-Pygments==2.6.1
+Pygments==2.7.4
 pyaml==20.4.0
 docker==4.2.0
 ruamel.yaml==0.16.10