Add black hook to CircleCI config

.circleci/config.yml

@@ -33,6 +33,12 @@ jobs:
             - .tox/py*
             - /root/.cache/pip/wheels/
 
+      - run:
+          name: Black style check
+          command: |
+            pip3 install --user black
+            python3 -m black --check /home/mythril/
+
       - run:
           background: true
           name: Launch of background geth instance