add pytest-cov

Author: Lionel Untereiner

poetry.lock

@@ -58,6 +58,7 @@ pylint = "^2.11.1"
 black = {version = "^21.11b1", allow-prereleases = true}
 flake8 = "^4.0.1"
 mypy = "^0.981"
+pytest-cov = "^4.0.0"
 
 [tool.black]
 line-length = 79