Add BytesIO support

What I'm saying is that I used to do this

img_bytes = pio.to_image(fig, format="svg", width=width, height=height)

and now I have to do the following, where k is an instance of Kaleido:

    img_bytes = await k.calc_fig(
        fig,
        opts={"format": "svg", "width": width, "height": height},
    )

I'm not trying to write the images to a file.

I'm not sure whether the docstring for calc_fig(), claiming "this function does not support parallelism", is accurate.

Originally posted by @s-banach in #231

Start by reading the implementation and docstring for calc_fig(), the entry point named in the issue, and trace how image output is currently returned or written. Clarify the expected BytesIO interface and parallelism behavior before changing it; done means callers can obtain image data in memory without writing a file and the documented behavior is accurate.