Add pre-commit hooks

- black
- flake8

See https://pre-commit.com

.pre-commit-config.yaml

+repos:
+  - repo: https://github.com/ambv/black
+    rev: stable
+    hooks:
+      - id: black
+        language_version: python3.6
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v1.3.0
+    hooks:
+      - id: flake8

pyproject.toml

+[tool.black]
+py36 = true