Upgrades

• CPython 3.15.0a2 -> 3.15.0a3

Build system changes

• The script used to start the build is now named build.py use, uv run build.py ... to start a build on all platforms.
• LLVM 21 is used to build all distribution, including those which target musl.

Distribution changes

• The startup logic (getpath) to better locate the install when invoked with an incorrect argv[0] was expanded to Python 3.11+

Full Changelog: 2025120...2025121