Deprecate unused AddList.

It's actually still used in get_vsize_hsize, but that is also currently
deprecated.  Also, there's a reasonable replacement.  The replacement
may be very slightly less efficient because it constructs deeply nested
Add objects, but given the low level of maintenance for axes_grid1,
decreasing the public API still seems a good idea.

Author: anntzer

doc/api/next_api_changes/removals/22465-AL.rst

@@ -0,0 +1,4 @@
+``mpl_toolkits.axes_grid1.axes_size.AddList``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... is deprecated due to being unused.  Use ``sum(sizes, start=Fixed(0))`` (for
+example) to sum multiple size objects.

lib/mpl_toolkits/axes_grid1/axes_size.py

@@ -16,7 +16,6 @@ class (or others) to determine the size of each axes. The unit
 
 
 class _Base:
-
     def __rmul__(self, other):
         return Fraction(other, self)
 
@@ -38,6 +37,8 @@ def get_size(self, renderer):
         return a_rel_size + b_rel_size, a_abs_size + b_abs_size
 
 
+@_api.deprecated(
+    "3.6", alternative="sum(sizes, start=Fixed(0))")
 class AddList(_Base):
     def __init__(self, add_list):
         self._list = add_list