Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')

This is happening because actions/setup-python is using Python 3.13 binary (latest version available in ubuntu-latest, recently updated to 24.04) and 'imghdr' was removed in Python 3.13 (See imghdr in 3.12 docs).

Pinning actions/setup-python to use Python 3.12 is the easiest way out.