Fix linkcheck

[hugovk]

We've been getting some rate limiting checking stackoverflow.com, resulting in a 10-minute timeout. Let's ignore it.

Plus fix the XZ link, the old one was removed because it was created by the backdoor author.

...
(getting-started/setup-building: line  780) broken    https://xz.tukaani.org/xz-utils/ - HTTPSConnectionPool(host='xz.tukaani.org', port=443): Max retries exceeded with url: /xz-utils/ (Caused by NameResolutionError("<urllib3.connection.HTTPSConnection object at 0x7f2b90d6b200>: Failed to resolve 'xz.tukaani.org' ([Errno -2] Name or service not known)"))
(developer-workflow/development-cycle: line  234) ok        https://github.com/python/devguide
(   triage/labels: line   33) ok        https://github.com/python/cpython/labels/type-security
(documentation/devguide: line   17) ok        [https://github.com/python/devguide/issues](https://github.com/python/devguide/issues?q=sort%3Aupdated-desc+is%3Aissue+is%3Aopen)
(documentation/translating: line    9) ok        https://github.com/python/docsbuild-scripts/
-rate limited-   https://stackoverflow.com/ | sleeping...
-rate limited-   https://stackoverflow.com/ | sleeping...
-rate limited-   https://stackoverflow.com/ | sleeping...
Error: The operation was canceled.

https://github.com/python/devguide/actions/runs/10852890901/job/30119946956?pr=1397

---

📚 Documentation preview 📚: https://cpython-devguide--1398.org.readthedocs.build/