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)

pyproject.toml

@@ -8,7 +8,7 @@ name = "aeg"
 dynamic = ["version"]
 description = "AEGIS encryption easy to use Python binding. Wheels for major platforms."
 readme = {file = "README.md", content-type = "text/markdown"}
-requires-python = ">=3.10"
+requires-python = ">=3.12"
 classifiers = [
   "Development Status :: 5 - Production/Stable",
   "Programming Language :: Python :: Implementation :: CPython",