fix AxisLabel not picking up fontsize from set_ylabel in axisartist - #31155

Fixes #28124.

When using host_subplot (or any axisartist.Axes), calling ax.set_ylabel('text', fontsize=20) sets the fontsize on the underlying yaxis.label, but the AxisLabel in axis_artist never picks it up — it only copies the text string and color from the ref artist. So the label renders at the default size regardless of what you pass to set_ylabel.

The fix adds get_fontsize/set_fontsize overrides to AxisLabel that inherit fontsize from the ref artist by default (similar to how get_color already works via AttributeCopier). If you set fontsize directly on the AxisLabel itself, the override is respected.

from mpl_toolkits.axisartist import Axes

fig = plt.figure()
ax = fig.add_subplot(111, axes_class=Axes)
ax.set_ylabel('ylabel', fontsize=20)
# before: label renders at default size (~10pt)
# after: label renders at 20pt as expected