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)

src/aeg/util.py

@@ -4,19 +4,12 @@ Currently provides Python-side aligned allocation helpers that avoid relying
 on libc/posix_memalign. Memory is owned by Python; C code only borrows it.
 """
 
-from typing import Protocol
+from collections.abc import Buffer
 
 from ._loader import ffi
 
 __all__ = ["new_aligned_struct", "aligned_address", "Buffer", "nonce_increment", "wipe"]
 
-try:
-    from collections.abc import Buffer  # type: ignore
-except ImportError:
-    # Fallback for Python < 3.12
-    class Buffer(Protocol):
-        def __buffer__(self, flags: int) -> memoryview: ...
-
 
 def aligned_address(obj) -> int:
     """Return the integer address of the start of a cffi array object."""