Docs/poetry: set version for sphinx-rtd-theme

While updating my nix-based environment to use poetry (through poetry2nix), I came across the known following bug https://github.com/readthedocs/sphinx_rtd_theme/issues/946 which prevents the installation of sphinx-rtd-theme from source.

This commit simply fixes sphinx-rtd-theme version to the previous release (0.4.3). There is a case to made that fixing dependencies is better for reproducibility anyhow.

Reviewers

@dannywillems @arvidnl