add dataclasses as a dependency for python < 3.7

Author: Fredrik Borg

pyproject.toml

@@ -17,6 +17,7 @@ orjson = "^3.2.1"
 shortuuid = "^1.0.1"
 wrapt = "^1.12.1"
 cachetools = "^4.1.1"
+dataclasses = {version = "^0.8.0", python = "<3.7"}
 
 [tool.poetry.dev-dependencies]
 pytest = "^6.2"