Pin `nltk>=3.9.3` when released, drop ignore

There's a vulnerability, CVE-2025-14009, in nltk<=3.9.2. It's been fixed in nltk's develop branch, but not yet released.

We've pinned our packages to the nltk's develop branch for now, but we don't want to release anything until we can pin to nltk>=3.9.3. We're also ignoring CVE-2025-14009 in the shared/Makefile security target until then.

Once nltk 3.9.3 is released, add the pin and drop the ignore.