lang/python315: update to 3.15.0b2 beta release

Specific updates since beta1 are in this
Changelog: https://docs.python.org/3.15/whatsnew/changelog.html#python-3-15-0-beta-2
Announcement: https://blog.python.org/2026/06/python-3150-beta-2/
What's new: https://docs.python.org/3.15/whatsnew/3.15.html

PR: 295830

Pull Request: https://github.com/freebsd/freebsd-ports/pull/529