Instead of

plt.subplot_mosaic(
    """
    AB
    CC
    """)

should we also allow a short string version

plt.subplot_mosaic("AB|CC")

?

Implementing is probably as simple as:

if '|' in layout and not '\n' in layout:
    layout = layout.replace('|', '\n')

to get back to the long version.

I got the idea while trying to find a compact replacement for the plot in https://matplotlib.org/devdocs/gallery/subplots_axes_and_figures/axes_margins.html.