This seems intended at easing fuzzy matching with trivial edits in the
msgstr (changing case and adding whitespace), but it was only done on
the new msgstr, not on the old mgstr candidates, so it was possible for
merging catalogs to miss messages.

difflib has a heuristic that used to make fuzzy matching unreliable for
>200char strings.  See

python/cpython#90825

Fixes #969