GHSA-6ww7-3frv-cqxh: NLTK: pathsec SSRF protection can be bypassed when a proxy is configured

### Summary Current NLTK source reopens SSRF in proxied environments. `pathsec.urlopen()` validates the requested hostname locally, but once proxy inheritance is enabled the real fetch is performed by the proxy rather than by the validated direct-connect socket path. ### Details - **Vulnerability type:** Server-side request forgery - **Affected component:** `nltk.pathsec.urlopen`, `nltk.data.load`, `nltk.downloader.Downloader.index`, `nltk.downloader.Downloader.download` - **Affected versions:** Current source `v3.10.0-rc2`; published `3.9.4` was a negative control and did not reproduce. …