[cffi] Annotate _CDataBase.__enter__ return type (#15701)

Author: pacman82

stubs/cffi/_cffi_backend.pyi

@@ -68,7 +68,7 @@ class _CDataBase:
     def __complex__(self) -> complex: ...
     def __delitem__(self, other, /) -> None: ...
     def __dir__(self): ...
-    def __enter__(self): ...
+    def __enter__(self) -> Self: ...
     def __eq__(self, other, /): ...
     def __exit__(
         self, type: type[BaseException] | None, value: BaseException | None, traceback: types.TracebackType | None, /