Use SHA256 from GitHub API for Python downloads (#14708)
We recently ran over the file limit and had to drop hash file from the
releases page in favor of bulk SHA256SUMS files
(https://github.com/astral-sh/python-build-standalone/pull/691).
Conveniently, GitHub has recently started to add a SHA256 digest to the
API. GitHub did not backfill the hashes for the old releases, so use the
API hashes for newer assets, and eventually only download SHA256SUMS for
older releases.
