Struct class lives in _struct (#12980)

This one is an improvement on 3.9+. On 3.8, the Struct class
calls itself `builtins.Struct` instead, which we can't and won't
match. `struct.error` is defined in `_struct.c`, but always called
itself `struct.error`.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

stdlib/_struct.pyi

@@ -0,0 +1,22 @@
+from _typeshed import ReadableBuffer, WriteableBuffer
+from collections.abc import Iterator
+from typing import Any
+
+def pack(fmt: str | bytes, /, *v: Any) -> bytes: ...
+def pack_into(fmt: str | bytes, buffer: WriteableBuffer, offset: int, /, *v: Any) -> None: ...
+def unpack(format: str | bytes, buffer: ReadableBuffer, /) -> tuple[Any, ...]: ...
+def unpack_from(format: str | bytes, /, buffer: ReadableBuffer, offset: int = 0) -> tuple[Any, ...]: ...
+def iter_unpack(format: str | bytes, buffer: ReadableBuffer, /) -> Iterator[tuple[Any, ...]]: ...
+def calcsize(format: str | bytes, /) -> int: ...
+
+class Struct:
+    @property
+    def format(self) -> str: ...
+    @property
+    def size(self) -> int: ...
+    def __init__(self, format: str | bytes) -> None: ...
+    def pack(self, *v: Any) -> bytes: ...
+    def pack_into(self, buffer: WriteableBuffer, offset: int, *v: Any) -> None: ...
+    def unpack(self, buffer: ReadableBuffer, /) -> tuple[Any, ...]: ...
+    def unpack_from(self, buffer: ReadableBuffer, offset: int = 0) -> tuple[Any, ...]: ...
+    def iter_unpack(self, buffer: ReadableBuffer, /) -> Iterator[tuple[Any, ...]]: ...