Add black formatter pre-commit hook and remove isort (since black also sorts imports)

Author: Daniel Gallagher

.pre-commit-config.yaml

@@ -22,11 +22,8 @@ repos:
     rev: v1.2.0
     hooks:
     -   id: pyupgrade
--   repo: https://github.com/asottile/seed-isort-config
-    rev: v1.0.1
+-   repo: https://github.com/ambv/black
+    rev: stable
     hooks:
-    -   id: seed-isort-config
--   repo: https://github.com/pre-commit/mirrors-isort
-    rev: v4.3.4
-    hooks:
-    -   id: isort
+    - id: black
+      language_version: python3.6