Bump pygments from 2.4.2 to 2.7.4 in /requirements (#7886)

Bumps [pygments](https://github.com/pygments/pygments) from 2.4.2 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.4.2...2.7.4)

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

requirements/requirements-optionals.txt

@@ -2,7 +2,7 @@
 psycopg2-binary>=2.8.5, <2.9
 markdown==3.3;python_version>="3.6"
 markdown==3.2.2;python_version=="3.5"
-pygments==2.4.2
+pygments==2.7.4
 django-guardian==2.2.0
 django-filter>=2.2.0, <2.3
 coreapi==2.3.1