Add CPython 3.15.0a6 (#17987)

Update available python versions to include CPython 3.15.0a6
Update scripts used to sync pbs releases to account for recent changes
in the repo and to support running on macOS.

crates/uv-python/src/sysconfig/generated_mappings.rs

@@ -1,7 +1,7 @@
 //! DO NOT EDIT
 //!
 //! Generated with `cargo run dev generate-sysconfig-metadata`
-//! Targets from <https://github.com/astral-sh/python-build-standalone/blob/20260203/cpython-unix/targets.yml>
+//! Targets from <https://github.com/astral-sh/python-build-standalone/blob/20260211/cpython-unix/targets.yml>
 //!
 #![allow(clippy::all)]
 #![cfg_attr(any(), rustfmt::skip)]