Add nbytes to the memoryview definition (#3167)

`nbytes`, like `contiguous`, was added in Python 3.3.

stdlib/2/__builtin__.pyi

@@ -784,6 +784,7 @@ class memoryview(Sized, Container[_mv_container_type]):
         c_contiguous: bool
         f_contiguous: bool
         contiguous: bool
+        nbytes: int
         def __init__(self, obj: Union[bytes, bytearray, memoryview]) -> None: ...
         def __enter__(self) -> memoryview: ...
         def __exit__(self, exc_type: Optional[Type[BaseException]], exc_val: Optional[BaseException], exc_tb: Optional[TracebackType]) -> bool: ...