Add black

Author: mana-ysh

{{ cookiecutter.project_slug }}/pyproject.toml

@@ -16,6 +16,7 @@ flake8 = "^3.7.9"
 mypy = "^0.770"
 pytest = "^5.4.1"
 pytest-cov = "^2.8.1"
+black = "^19.10b0"
 
 [build-system]
 requires = ["poetry>=0.12"]