Add black to requirements file.

tim-schilling · tim-schilling · commit 479a356a103d · 2020-09-18T11:26:56.000-05:00
It's necessary to run 'make sort' so it should be in the
dependencies.
diff --git a/requirements_dev.txt b/requirements_dev.txt
@@ -12,6 +12,7 @@ html5lib
 isort
 selenium
 tox
+black
 
 # Documentation
 
