Added dataclasses to dev-dependencies to work around a Poetry issue

Author: kierandarcy

poetry.lock

@@ -32,6 +32,8 @@ mypy = "0.720"
 black = "20.8b1"
 pre-commit = "~1.17.0"
 tox-gh-actions = "^1.3.0"
+# https://github.com/python-poetry/poetry/issues/2386
+dataclasses = { version = "^0.7", python = "^3.6.1, <3.7" }
 
 [build-system]
 requires = ["poetry>=0.12"]