Before 3.6.0, Colorbars for mappables that do not have a parent Axes would
steal space from the current Axes. 3.6.0 raised an error on this, but without a
deprecation cycle. For 3.6.1 this is reverted; the current Axes is used, but a
deprecation warning is shown instead. In this undetermined case, users and
libraries should explicitly specify what Axes they want space to be stolen
from: fig.colorbar(mappable, ax=plt.gca()).