API Changes for 3.4.2

Behaviour changes

Rename first argument to subplot_mosaic

Both FigureBase.subplot_mosaic, and pyplot.subplot_mosaic have had the first position argument renamed from layout to mosaic. This is because we are considering to consolidate constrained_layout and tight_layout keyword arguments in the Figure creation functions of pyplot into a single layout keyword argument which would collide.

As this API is provisional, we are changing this with no deprecation period.