gh-59598: Ignore leading whitespace in `JSONDecoder.raw_decode`

[crazymerlyn]

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Based on Bayard's patch in https://bugs.python.org/issue15393 Fixes #59598

• Issue: gh-59598

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[crazymerlyn]

Updated the code to make the matching function an argument and removed the extra call.

[eendebakpt]

@crazymerlyn Thanks for the updates. Your pr still requires a news entry. The easiest way to add this is to use click on the Details button next to the failing bedevere/news in the list of CI checks.

Also see https://devguide.python.org/core-developers/committing/#how-to-add-a-news-entry

[crazymerlyn]

added a news entry

[crazymerlyn]

@eendebakpt Updated with the suggested change.

[eendebakpt]

@crazymerlyn I read the corresponding issue and opinions on whether we should make this change or not (at least back in 2016!) are not unanimous. A core developer should decide whether to accept this or not. The code in the PR looks good to me.

Since this PR is not backwards compatible and there has been some discussion it is good to update the documentation (e.g. here https://docs.python.org/3/library/json.html#json.JSONDecoder.raw_decode) and add a Changed in version x.xx section (for an example see https://docs.python.org/3/library/bisect.html#bisect.bisect)