Instead of accepting any str, we only accept as set of predefined literals. For simplicity, we don't distinguish the between allowed positions for Axes legend and figure legend. It's still better to limit the allowed range to the union of both rather than to accept abitrary strings.

This is a bit cumbersome, but since loc names are hard to remember and easy to misspell, I believe it's a real benefit if in-editor type checkers could notify on an invalid loc name.