Prepare Python SDK 2.3.7 history-page release

[rmcdaniel]

Release

Prepare Python SDK 2.3.7 for the bounded worker history poll fix in #88. This patch updates the package, product-train and registry versions and changelog. It does not change the worker protocol.

Qualification before tag

• Fix PR Bound Python worker history polls for long workflows #88 passed its current-head Python 3.10, 3.11 and 3.12, lint, integration, package, regression-corpus, docs and public-boundary checks.
• A worker with the candidate source recovered the exact 6,529-event run that failed under published SDK 2.3.6, on unchanged published Server 2.4.26. It completed 4,000 signals, 4,000 side effects, eight activity/timer boundaries and continuation with the exact result. Commands and raw evidence.
• This release PR must pass its own current-head CI before tagging.

After publication

Verify immutable tag, PyPI wheel/sdist and metadata. Repeat the long mixed fixture from an installed Python SDK 2.3.7 against the same exact published Server image, then run affected published replay conformance. Keep #87 open until the published package proves the outcome.

Server, PHP SDK, Rust SDK, CLI and Waterline do not embed the Python package, so this patch does not require their rebuild. PHP and Rust independently need bounded poll requests and are tracked in their owning issues. Python documentation deploys from this repository after merge.