Python 3.15 readiness: pre-release CI leg and the argparse 3.15 behavior changes

[luisgf]

Horizon: short · Effort: S-M · 2026-07-17 roadmap analysis (round 3)

Python 3.15.0 final is scheduled for 2026-10-01 (PEP 790; rc1 on 2026-08-04) — the same window as the v4.0.0 release. Get ahead of it instead of debugging it in release week:

• add an allow-failure CI leg on the 3.15 release candidate once rc1 lands (2026-08-04)
• argparse in 3.15 turns on suggest_on_error by default and colorizes help — review the CLI tests that compare usage/help/error text so they hold on 3.15, and decide whether cli_common pins these options for stable output across versions
• 3.15's hashlib always exposes the guaranteed hash constructors (probing via AttributeError no longer fires) — grep for availability probes relying on that pattern; expected none
• when 3.15.0 final is out: full matrix leg + Programming Language :: Python :: 3.15 classifier — ideally inside v4.0.0 (v4.0.0: single grouped breaking release (tracking) #170) so the major ships supporting 3.11–3.15
• PyLD note: [ldp] already resolves PyLD 3.1.0 green in CI, but PyLD publishes no explicit 3.15 support statement and leans on lxml wheels — the rc leg is what surfaces a problem early

Refs #170.