UnspacedList._spaced_position: support the slice at the end fo the list

- Which is needed for .insert()ing at the end, for instance.

certbot-nginx/certbot_nginx/nginxparser.py

@@ -279,6 +279,9 @@ class UnspacedList(list):
         # Normalize indexes like list[-1] etc, and save the result
         if idx < 0:
             idx = len(self) + idx
+        if idx == len(self):
+            # not an index, but the slice at the end of the list
+            return len(self.spaced)
         if not 0 <= idx < len(self):
             raise IndexError("list index out of range")
         idx0 = idx