Make stale bot use "abandoned" instead of "wontfix"

Fixes #630

.github/stale.yml

@@ -7,7 +7,7 @@ exemptLabels:
   - pinned
   - security
 # Label to use when marking an issue as stale
-staleLabel: wontfix
+staleLabel: abandoned
 # Comment to post when marking an issue as stale. Set to `false` to disable
 markComment: >
   This issue has been automatically marked as stale because it has not had