Fix pluralize for words ending in z

[Debasishhh]

The change

pluralize() added a bare s to words ending in z:

>>> pluralize('buzz'), pluralize('waltz'), pluralize('quiz')
('buzzs', 'waltzs', 'quizs')

#429 added x to the -es rule but z was missed. This adds z to that rule (buzzes, waltzes, Fizzes), and adds quiz to the irregular plurals so it becomes quizzes (and singularize('quizzes') returns quiz). A general z-doubling rule would be wrong for words like topaz, so quiz is listed explicitly.

The backstory

Follow-up to #585, found during the same read-through.

AI assistance

• Harness/tooling (Claude Code, Cursor, Codex, etc.): Claude Code
• Model(s): Claude Opus 5.5 (Sonnet subagents for the issue search)
• Original prompt(s), as close to verbatim as you can manage:
  • "use this skill to find out repos i can contribute" (a code-reading pass surfaced the pluralize bug)
  • "now do the pluralize fix"

[Wkhanex]

There is an adjacent singularize gap worth considering alongside the new z plural rule. At b4c338e, the new plurals are correct, but feeding them into the paired helper gives:

singularize(pluralize('buzz'))   # 'buzze'
singularize(pluralize('waltz'))  # 'waltze'
singularize(pluralize('topaz'))  # 'topaze'

The same issue affects fizz and is preserved in title/upper case, while the new irregular quiz entry round-trips correctly. Would it make sense to extend the reverse -zes handling and cover these round trips too? The singularize behavior for these already-plural strings predates this PR; this is an adjacent consistency gap, not a claim that the PR introduced that implementation defect.

I ran an independent expected-word/case matrix and the existing strutils tests on CPython 3.12.14. All 28 existing tests pass; the added round-trip check fails with the results above.

Review prepared with Codex assistance; the checks described above were executed locally.

[Debasishhh]

Yeah singularize is already off for these, boxes gives boxe too. Didn't want to mix it in here though since -zes is ambiguous (buzzes -> buzz but sizes -> size). Can do a separate PR if @mahmoud wants