Fix third-party workflow

[AlexWaygood]

pdm now won't let us add the main branch of typing_extensions as a dependency to pydantic or cattrs, because they both still have requires-python = ">= 3.7" in their pyproject.toml files, and we now have requires-python = ">= 3.8" in ours. To me it seems silly that pdm's locking mechanism is working this way, since we're not trying to actually install or run pydantic or cattrs on Python 3.7. But anyway, here's a hacky workaround to get things green again.

Fixes #268

[AlexWaygood]

To me it seems silly that pdm's locking mechanism is working this way, since we're not trying to actually install or run pydantic or cattrs on Python 3.7.

Oh, I guess it's so that pip install pydantic is guaranteed to come with the same version of typing_extensions for any given version of pydantic, meaning that you get fully reproducible installs. I guess that makes sense. Still annoying for us, though.