Re-pin python-build to 20260714: iOS _posixshmem restored (multiprocessing imports)

[FeodorFitsner]

Re-pins the bundled python-build snapshot from 20260712 to 20260714 within the still-unreleased 4.3.2 (no version bump; package versions are already 4.3.2 from #230).

The snapshot delivers flet-dev/python-build#30, an iOS-only fix: the _posixshmem extension is built into the iOS runtimes again. multiprocessing.resource_tracker — imported transitively by import multiprocessing (e.g. scikit-learn → joblib) — unconditionally imports _posixshmem on posix, so with _multiprocessing enabled (since 20260701) but _posixshmem missing, any multiprocessing user died with ModuleNotFoundError: No module named '_posixshmem'. Process spawning remains unsupported in the iOS sandbox — this only makes the shared-memory module importable. Python (3.12.13 / 3.13.14 / 3.14.6) and dart_bridge (1.5.0) versions are unchanged.

Changes:

• Version tables regenerated with dart run serious_python:gen_version_tables --release-date 20260714 (only the release date moved); verified idempotent with the no-arg form.
• 4.3.2 CHANGELOG entries updated: snapshot references now read 20260714, with the _posixshmem fix added to serious_python and serious_python_darwin.