linkcheck: update implementation to make use of py311 features

[jayaddison]

Feature or Bugfix

• Refactoring (mostly; does introduce small behaviour changes)

Purpose

• Use StrEnum values to provide type-checkable linkcheck status codes.

Detail

• Utilises Python 3.11, for StrEnum and Structural Pattern Matching support (ref Drop support for Python 3.10 #13000).
• Removes apparently unused status codes (local, old).

Relates

• Reinstantiates Draft: linkcheck builder: refactor status codes into an enum #11865.

[AA-Turner]

Could you elaborate on the case for using StrEnum over plain strings? Can we use match/case with literal values?

I'd prefer to split the changes to use match-case and to use an enum, as they can be done separately, and it reduces the diff.

A

[jayaddison]

Could you elaborate on the case for using StrEnum over plain strings?

The main benefit I'm seeking with that change is to reduce the chance that a typo/change to a status code in one part of the code isn't caught by typechecking / code review -- currently they are string literals, so the reader has to be aware that they are used for equality checks and must be kept consistent/equal.

Can we use match/case with literal values?

Yep, that would be possible. I would personally prefer to apply the enum refactoring first, because it introduces the kind of symbol-strictness that could catch problems in a refactor that involves string literals.

I'd prefer to split the changes to use match-case and to use an enum, as they can be done separately, and it reduces the diff.

Makes sense; I think I've been a bit hasty putting this pull request together. I'll begin afresh with a simpler, enum-only refactor soon (within the next week or so) if that sounds reasonable. I'd like to get that in place before an implementation of #12985.

Edit: s/seeing/seeking -- typo with change-of-interpretation (!)

[AA-Turner]

That seems reasonable. In the interim we could tighten up the static typing from str to a Literal which would achieve many of the same goals, I think.

A