…ues (#3541)

### Summary

Bumps to `nltk==3.9.1` and resolves
[CVE-2024-39705](https://nvd.nist.gov/vuln/detail/CVE-2024-39705). An
NLTK version bump was originally introduced in #3512 and rolled back in
#3527 because `nltk==3.8.2` was yanked from PyPI, and also because we
observed significant slowdowns in processing time after bumping to
`nltk==3.8.2`. The processing time regression does not appear in
`nltk==3.9.1`.

### Testing

After the bump, CI should pass. Additionally we verified locally that
files processing takes around the amount of time we would expect for a
long `.docx` file.

```python
In [1]: from unstructured.partition.auto import partition

In [2]: filename = "test-doc.docx"

In [3]: %timeit partition(filename=filename)
3.92 s ± 73 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```