Add typing_extensions for python_version < '3.8'

pyproject.toml

@@ -7,7 +7,7 @@ name = "conda-join"
 description = "Unified Conda and Pip requirements management."
 dynamic = ["version"]
 authors = [{ name = "Bas Nijholt", email = "[email protected]" }]
-dependencies = ["ruamel.yaml"]
+dependencies = ["ruamel.yaml", "typing_extensions; python_version < '3.8'"]
 requires-python = ">=3.7"
 
 [project.readme]