Fix ReadTheDocs CI

[edrevo]

CI is failing due to a bug in a recently (2h ago) released version of snowballstemmer. This PR pins the version to the 2.x release to avoid the bug. See snowballstem/snowball#229 for more information.

[github-actions]

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[DarkLight1337]

Thanks for fixing! However we have already opened #17848 so we'll use that instead.