Make FFI.buffer implement buffer protocol, mark dunder methods as positional-only (#14505)

stubs/cffi/_cffi_backend.pyi

@@ -90,16 +90,17 @@ class _CDataBase:
 class buffer:
     __hash__: ClassVar[None]  # type: ignore[assignment]
     def __init__(self, *args, **kwargs) -> None: ...
-    def __delitem__(self, other) -> None: ...
-    def __eq__(self, other): ...
-    def __ge__(self, other): ...
-    def __getitem__(self, index): ...
-    def __gt__(self, other): ...
-    def __le__(self, other): ...
+    def __buffer__(self, flags: int, /) -> memoryview: ...
+    def __delitem__(self, other, /) -> None: ...
+    def __eq__(self, other, /): ...
+    def __ge__(self, other, /): ...
+    def __getitem__(self, index, /): ...
+    def __gt__(self, other, /): ...
+    def __le__(self, other, /): ...
     def __len__(self) -> int: ...
-    def __lt__(self, other): ...
-    def __ne__(self, other): ...
-    def __setitem__(self, index, object) -> None: ...
+    def __lt__(self, other, /): ...
+    def __ne__(self, other, /): ...
+    def __setitem__(self, index, object, /) -> None: ...
 
 # These aliases are to work around pyright complaints.
 # Pyright doesn't like it when a class object is defined as an alias