chore: ensure nltk version is >=3.9.1 to fix security issue

This reverts the version pin added in commit:
bfc5f32
This can be done now since some of our dependencies,
such as `valentine` have upgraded as well. See pull
request delftdata/valentine#75.
The minimum version requirement for valentine is needed
because the ntlk upgrade is a breaking change: using
valentine with older nltk would cause bugs related to
the punkt tokenizer.

requirements.txt

@@ -1,5 +1,5 @@
 polyfuzz
-valentine
+valentine>0.2.0
 openai
 torch
 transformers
@@ -13,4 +13,4 @@ panel!=1.4.3
 Levenshtein
 autofj
 conllu<5.0.0
-nltk<3.8.2
+nltk>=3.9.1