[dep] #90, #103: Introduce pytest and pytest-cov dev dependencies

pyproject.toml

@@ -36,6 +36,8 @@ graphql-core = "^3.1.2"
 websocket-client = "^0.57"
 
 [tool.poetry.dev-dependencies]
+pytest = "^6.2.3"
+pytest-cov = "^2.12.0"
 black = {version = "^21.4b2", allow-prereleases = true}
 mypy = "^0.790"
 pylint = "^2.7.2"