feat: make a PNG export of the figure available in the kernel

[mariobuikhuizen]

A solution for #1393

Usage:

def on_png_received(data):
    with open('mychart.png', 'bw') as f:
        f.write(data)

figure.get_png_data(on_png_received)

[maartenbreddels]

LGTM, would like a 2nd opinion from @SylvainCorlay or @martinRenou on this.

Some thoughts that don't need addressing perse:

• download vs upload depends on your perspective, maybe we can avoid that word, maybe call the event 'send_msg'?
• This particular use case is asking for a await fig.get_png_data_async(), which might work with @davidbrochart 's https://github.com/davidbrochart/akernel

[martinRenou]

Rebased

[martinRenou]

@meeseeksdev backport to 0.12.x

[pllim]

Thanks! Nice! Which release will this feature be in? 😸

[martinRenou]

0.12.31 :) It should come soon

[pllim]

Seems to work, though this thing gets stuck in display-less CI, so I cannot write a test for it. Any trick I need to know to write a Python unit test that calls this function?