cbook removals

• matplotlib.cbook.MatplotlibDeprecationWarning and matplotlib.cbook.mplDeprecation are removed; use matplotlib.MatplotlibDeprecationWarning instead.

• cbook.maxdict; use the standard library functools.lru_cache instead.

Groupers from get_shared_x_axes / get_shared_y_axes are immutable

Modifications to the Groupers returned by get_shared_x_axes and get_shared_y_axes are no longer allowed. Note that previously, calling e.g. join() would already fail to set up the correct structures for sharing axes; use Axes.sharex or Axes.sharey instead.