Improve `{Readable, WriteableBuffer}Buffer`

[elpekenin]

Right now, there is code using len() on variables hinted as such types. However, this is not correct with the current implementation of hints due to the lack of Sized.

I believe they should be slightly changed like this, such that the type correctly marks the existence of __len__(self) -> int

ReadableBuffer: TypeAlias = TypeVar(
    "ReadableBuffer",
    Sized,
    bound=Union[
        array.array,
        bytearray,
        bytes,
        memoryview,
        "rgbmatrix.RGBMatrix",
        "ulab.numpy.ndarray",
    ],
)

But im not sure about this syntax, as im not a typing expert, would love some feedback before raising a PR. Thanks in advance :)