Add to build-system to pyproject.toml for readthedocs builds.
@chill I had to add the following to pyproject.toml to get readthedocs to build documentation.
[build-system] requires = ["poetry_core>=1.0.0"] build-backend = "poetry.core.masonry.api"
see: https://docs.readthedocs.io/en/stable/guides/poetry.html?highlight=poetry