Install black before trying to use it.

Author: gaffney2010

.github/workflows/config.yml

@@ -33,6 +33,12 @@ jobs:
         python -m pip install docutils==0.12
         python -m pip install mock
         cd docs; make clean; make html; cd ..;
+    - name: Install testing dependencies
+      run: |
+        python -m pip install coverage
+        python -m pip install hypothesis==3.2
+        python -m pip install mypy
+        python -m pip install black
     - name: Run tests
       run: |
         python -m pip install coverage