config.types: add SetAttribute type, based on ListAttribute

[dgw]

Description

Implements the new config type idea from #2687 (comment)

Most of the parsing/serializing code is just inherited, with a thin layer of tweaks on top to return the correct type.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make lint and make test)
  • I didn't have a chance to lint, sadly. (I hate travel days, but I don't mind leaning on CI sometimes.)
• I have tested the functionality of the things this change touches
  • via tests, naturally

Notes

Draft until #2687 gets merged, because that's the first usage site for this and there's not much point in adding the feature without something in core that uses it.

[dgw]

After converting the .blocks lists to SetAttribute (needs #2687 merged first to avoid unnecessary merge conflicts), we should audit all the .blocks code again for usage of strip() or other likely-unnecessary processing steps that the SetAttribute itself should handle. (The existing ListAttribute code probably already makes those unnecessary, but I'm deferring the audit in the interest of—again—reducing PR churn because of conflicts.)

Edit: Also, use of str() casts as mentioned here:

Nit: Huh, do we expect not-str here for some reason? Wondering why this isn't:

But I guess this question extends to the other branches too since it's clearly written to match the existing code. Perhaps handling Identifier got a bit infectious here, or it's done in the name of strict symmetry?

[dgw]

More things to audit after converting to SetAttribute:

• if not bad_mask: continue (bot, coretasks, config: add hostmask block type #2687 (comment))
• Unnecessary local bad_masks in _hostmask_blocked() helper (bot, coretasks, config: add hostmask block type #2687 (comment))
• Allow each field of _is_pretrigger_blocked()'s returned tuple to be separately nullable, for greater semantic meaning (bot, coretasks, config: add hostmask block type #2687 (comment))

[Exirel]

Sure! 😁