Fix pytest-black version incompatibility

Author: kevin-brown

pyproject.toml

@@ -46,7 +46,7 @@ pytest-django = ">=4.1"
 pytest-cov = ">=2.10"
 pytest-isort = ">=1.2"
 pytest-flake8 = ">=1.0"
-pytest-black = ">=0.3"
+pytest-black = {version = ">=0.3", python = ">=3.6.2,<4"}
 freezegun = ">=1.0"
 coverage = {version = ">=5.0", extras = ["toml"]}
 celery = ">=4"