This seems to be an issue with the latest release of docutils. I just started seeing the exact same failure with docutils on my Github Actions builds (only on macOS, not Windows or Ubuntu). It can be reproduced with mamba create -c conda-forge -n env_name python docutils . Docutils 0.22.1 was released 2 days ago, and there's a ticket in their issue tracker about a problem with installation. I would pin docutils to the previous working version and wait for a fix.