Add dataclasses dependency.

For Python 3.6, which is what the Centos image has (where I build the
executable using Pyinstaller).

pyproject.toml

@@ -28,6 +28,7 @@ exclude = [
 [tool.poetry.dependencies]
 python = "^3.6.1"
 requests = "^2.25.1"
+dataclasses = { version = '^0.8', python = '<3.7'}
 
 [tool.poetry.dev-dependencies]
 pre-commit = "^2.13.0"