chore: Fix missing pytest-cov dependency

Author: pawamoy

poetry.lock

@@ -30,6 +30,7 @@ ipython = "^7.3"
 isort = { version = "^4.3", extras = ["pyproject"] }
 jinja2-cli = { git = "https://github.com/mattrobenolt/jinja2-cli.git" }
 pytest = "^4.3"
+pytest-cov = "^2.6"
 pytest-sugar = "^0.9.2"
 pytest-xdist = "^1.26"
 recommonmark = "^0.5.0"