#153: Allow to install black pre-releases

• black is only available as a pre-release
• Check is performed/The error appears from Poetry v1.1.5
• Fix CI jobs

Close #153 (closed).

pyproject.toml

@@ -35,7 +35,7 @@ graphql-core = "^3.1.2"
 websocket-client = "^0.57"
 
 [tool.poetry.dev-dependencies]
-black = "^20.8b1"
+black = {version = "^20.8b1", allow-prereleases = true}
 mypy = "^0.790"
 pylint = "^2.7.2"
 sphinx = "^3.2.1"