Update logo to reflect IPython.

[Carreau]

Especially in a context where Python can be used with Xeus, it is important to have a distinction. This updates the logos shown in (for example), JupyterLab to be clearer, but still convey "Python".

[Carreau]

cc @SylvainCorlay

[SylvainCorlay]

Thanks for closing. For reference, I recal we agreed to keep the language icon and indicate the kernel in the text.