Fixed #23530 -- Specified PyYAML requirement in initial-data.txt

Thanks aks for the report.
Fixed #23196 -- Short-circuited empty string translation

Translating an empty string used to return the gettext catalog
metadata instead of the empty string.
Thanks Ned Batchelder for the suggestion, Tim Graham for the review
and Anton Berezin and Claude Paroz for contributions to the patch.
Fixed an i18n test with native string

Refs commit 11f307a5a8.
Fixed #23529 -- Replaced comments tag library with humanize in docs
Fixed #23527 -- Removed Cheetah references in docs

Thanks to @aaugustin for submitting the ticket.
Fixed #23512 -- Made migration docs about initial_data clearer

Thanks @abhillman and @Markush2010 for submitting the ticket.