acceptable that a flat typed keymap with a dir_archive fails on windows?

[mmckerns]

```keymap(typed=True, flat=True) used with adir_archive` seems to always fail on Windows… or at least it should fail for the majority of keys. Likely due to certain "bad" characters in the directory name… which are allowable on non-windows platforms.

Should this be at least better documented? or otherwise throw a better error?
I expect this could also happen for certain choices of SENTINEL, and also for certain objects used as keys in the keymap. The easy fix is to use one of the other keymaps (like hashmap).