singlehtml: deprecate 'fix_refuris' helper function

[jayaddison]

Feature or Bugfix

• Deprecation

Purpose

• Deprecates a singlehtml builder helper method that I believe should be redundant since 🐛 Fix singlehtml target uris to be same-document references #11970 and singlehtml: Use same-document hyperlinks for internal project references #12551 are in place, but that might, perhaps, be re-used by other functionality (extensions / codebases).

Detail

• Deprecates the sphinx.builders.singlehtml.SingleFileHTMLBuilder.fix_refuris method, and adds a warning noting that it's planned for removal in Sphinx 9 (the next major version).

Relates

• Follow-up to 🐛 Fix singlehtml target uris to be same-document references #11970 and singlehtml: Use same-document hyperlinks for internal project references #12551.

Edit: update description: this PR removes calls to the method, so don't imply that this is deprecation of already-unused code.

[AA-Turner]

Remember to add new deprecations to doc/extdev/deprecated.rst.

A

[jayaddison]

Although I'm fairly keen to merge this, and think it should be safe -- maybe I'll take one more self-review pass (and/or try some builds of external singlehtml projects) on it next week before re-requesting approval/merge.