If one does not set any keymap settings in an rcParams, then the default keymaps are used from rcsetup.defaultParams. The validation function "validate_stringlist" will always return a list, however, some of the default values are given as just a string, not a list of strings. These values never go through validate_stringlist, and therefore, never get turned into a list.

Do we want to force a validation run on the defaults upon loading mpl, or do we want to just simply fix those entries to be lists? Note that there may be issues with users who have come to expect some of these entries to be strings.