Update to stock CPython 3.15.x when available

Now that https://github.com/python/cpython/pull/146140 is merged and will presumably be part of 3.15, we should be able to stop patching CPython and use a stock release. I believe we'll still need to build from source unless we can get "official" `libpython3.15.so` builds which include `libz`, `libsqlite3`, and sockets support.

Start by locating the current CPython patching and source-build entry points. Use CPython pull request 146140 as context, then verify whether a stock 3.15.x release provides libpython3.15.so with zlib, SQLite, and socket support. Done means the project uses the stock release where possible, or documents why a source build remains necessary.

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