clarify pygments version pinning

Context: https://github.com/encode/django-rest-framework/security/dependabot/9

Author: peterthomassen

requirements/requirements-optionals.txt

@@ -6,5 +6,5 @@ django-guardian>=2.4.0,<2.5
 inflection==0.5.1
 markdown>=3.3.7
 psycopg2-binary>=2.9.5,<2.10
-pygments>=2.12.0,<2.14.0
+pygments>=2.12.0,<2.14.0  # 2.15 dropped Python 3.6 support, can bump once we drop 3.6 as well
 pyyaml>=5.3.1,<5.4