Add black to requirements file.
Add black to requirements file.

It's necessary to run 'make sort' so it should be in the
dependencies.

tim-schilling/django-debug-toolbar,tim-schilling/django-debug-toolbar,spookylukey/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,spookylukey/django-debug-toolbar,jazzband/django-debug-toolbar,tim-schilling/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,jazzband/django-debug-toolbar,spookylukey/django-debug-toolbar,jazzband/django-debug-toolbar