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.