Mapping bpo numbers in what's new does not reference GitHub issues correctly.

[GadgetSteve]

In the document build process the code in Doc/tools/extensions/pyspecific.py maps BPO or # references to bugs.python.org which as been retired and replaced with Issues in https://github.com/python/cpython/issues/ however while the issues that existed at bugs.python.org all have been ported to https://github.com/python/cpython/issues/ the reference numbers are not the same so simply changing the URL does not map to the correct issue - e.g. BPO-45292 was https://bugs.python.org/issue45292 but is now #89455 i.e. 89455 not 45292

Possible approaches include:

1. Generate a mapping for all of the BPO entries and using that for all historic commits a large file with no other use
2. Check each issue to get the link from the BPO page to the GH page time consuming and likely to hit rate limits
3. Come up with a cut off number where all references below it are still mapped to BPO and those after to GH. may work but where items were opened on BPO and are now on GH may be misleading.
4. Come up with a tag, e.g. gh or PGH as an alternative to BPO and use the new URL for those - relies on all the users remembering to use it
5. Any references that are bare # without a BPO prefix reference the GitHub issue but map those with the prefix to BPO. Risk that some older issue references also just use # and are incorrectly mapped as a result.
6. See if there is a simple formula to transform - if all of the issues were transferred contiguously and in order there may be!
7. Combine 5 with 3 - less likely to hit both issues at once!

Some guidance/discussion on the preferred approach is needed.

[hugovk]

Ping @ezio-melotti

[GadgetSteve]

I notice that the developer guide recommends that a gh- prefix be added to all pull requests we could possibly use this to detect as in 4 above, possibly combined with a git off number for any bare # numbers.

[GadgetSteve]

Found that I can export from BPO to a .csv file and then parse it to a dictionary (7121 entries) of BPO numbers to GH numbers - the BPO numbers run from 1062 to 1776674 and the GH numbers from 34627 to 91417 to there is significant overlap - but only 41 collisions.
This might provide some possibilities.

[ezio-melotti]

This should already be fixed in #91454

[ezio-melotti]

bpo-* links in https://docs.python.org/3/whatsnew/changelog.html#changelog are now working, so this issue can be closed. If I missed anything let me know!

the BPO numbers run from 1062 to 1776674 and the GH numbers from 34627 to 91417 to there is significant overlap - but only 41 collisions.

bugs.python.org supports automatic redirects now, but I was thinking to expose an API to get the GitHub id from a bpo id (or even just the whole mapping) if it's useful. Note that issues with an high ID (>100k) are old issues that have been migrated from SourceForge when we switched to Roundup, and they kept their old ids. See psf/gh-migration#1 for more info.