chore(deps-dev): bump black from 24.3.0 to 26.3.1

Security cleanup tracked in DFPY-64.

Author: dependabot[bot]

requirements-dev.txt

@@ -4,7 +4,7 @@
 # Development and testing dependencies
 just==0.8.162
 isort==5.13.2
-black==24.3.0
+black==26.3.1
 blacken-docs==1.13.0
 certifi==2025.4.26
 flake8==7.0.0