Failure to build docs

[pombredanne]

Docs no longer build in the CI:

2025-05-08T10:25:27.8295777Z reading sources... [ 87%] rst_snippets/scancode-reindex-licenses
2025-05-08T10:25:27.8374915Z reading sources... [ 88%] rst_snippets/stdout
2025-05-08T10:25:27.8428397Z reading sources... [ 89%] rst_snippets/tip_snippets/stdout
2025-05-08T10:25:27.8489831Z reading sources... [ 90%] rst_snippets/warning_snippets/control_ignore_author_copyright
2025-05-08T10:25:27.8539392Z reading sources... [ 91%] rst_snippets/warning_snippets/output_htmlapp_dep
2025-05-08T10:25:27.8587211Z reading sources... [ 93%] tutorials/how_to_add_a_post_scan_plugin
2025-05-08T10:25:27.9003015Z reading sources... [ 94%] tutorials/how_to_extract_archives
2025-05-08T10:25:27.9094798Z reading sources... [ 95%] tutorials/how_to_format_scan_output
2025-05-08T10:25:27.9218661Z reading sources... [ 96%] tutorials/how_to_run_a_scan
2025-05-08T10:25:27.9381110Z reading sources... [ 97%] tutorials/how_to_set_what_will_be_detected_in_a_scan
2025-05-08T10:25:27.9632655Z reading sources... [ 98%] tutorials/how_to_visualize_scan_results
2025-05-08T10:25:27.9688317Z reading sources... [100%] tutorials/index
2025-05-08T10:25:27.9729830Z 
2025-05-08T10:25:27.9732150Z looking for now-outdated files... none found
2025-05-08T10:25:27.9802879Z pickling environment... done
2025-05-08T10:25:27.9805176Z checking consistency... done
2025-05-08T10:25:27.9807803Z preparing documents... failed
2025-05-08T10:25:27.9812335Z 
2025-05-08T10:25:27.9812521Z Exception occurred:
2025-05-08T10:25:27.9832538Z   File "/opt/hostedtoolcache/Python/3.12.10/x64/lib/python3.12/site-packages/snowballstemmer/__init__.py", line 27, in stemmer
2025-05-08T10:25:27.9833393Z     raise KeyError("Stemming algorithm '%s' not found" % lang)
2025-05-08T10:25:27.9834041Z KeyError: "Stemming algorithm 'porter' not found"
2025-05-08T10:25:27.9840357Z The full traceback has been saved in /tmp/sphinx-err-j4hnr2xm.log, if you want to report the issue to the developers.
2025-05-08T10:25:27.9841213Z Please also report this if it was a user error, so that a better error message can be provided next time.
2025-05-08T10:25:27.9841798Z A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
2025-05-08T10:25:28.0733070Z ##[error]Process completed with exit code 2.
2025-05-08T10:25:28.0832679Z Post job cleanup.