Require Python 3.12+; drop PyPy and 3.10/3.11 wheel builds

- requires-python >= 3.12; abi3 limited API tag cp310 -> cp312
- util.Buffer now imported directly from collections.abc (no fallback)
- release.py: ABI3 wheel covers 3.12-3.15; PyPy wheels dropped (PyPy
  has no 3.12 support yet)

setup.py

@@ -37,6 +37,6 @@ if __name__ == "__main__":
     setup(
         cffi_modules=["setup.py:ffibuilder"],
         options=(
-            {"bdist_wheel": {"py_limited_api": "cp310"}} if not is_free_threaded else {}
+            {"bdist_wheel": {"py_limited_api": "cp312"}} if not is_free_threaded else {}
         ),
     )