MNT: Deprecate cbook.maxdict

It was used in one place in the library and there is now
a standard library lru_cache that we can take advantage of instead.

Author: greglucas

doc/api/next_api_changes/deprecations/22323-GL.rst

@@ -0,0 +1,4 @@
+``matplotlib.cbook.maxdict`` is deprecated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This class has been deprecated in favor of the standard library
+``functools.lru_cache``.

lib/matplotlib/cbook/__init__.py

@@ -540,6 +540,7 @@ def flatten(seq, scalarp=is_scalar_or_string):
             yield from flatten(item, scalarp)
 
 
+@_api.deprecated("3.6", alternative="functools.lru_cache")
 class maxdict(dict):
     """
     A dictionary with a maximum size.