Fix traceback for top_edge == "L"

No longer access the t1 edge type after changing it which may lead
to t1 being a char instead of a Edge instance

Thanks to michael (https://hackaday.io/hacker/381347-michael) for
pointing this out!

boxes/lids.py

@@ -115,7 +115,7 @@ class _TopEdge(Boxes):
             t3 = "N"
         elif t1.char == "v":
             t2 = t3 = t4 = "e"
-        if t1.char == "t":
+        elif t1.char == "t":
             t1 = t3 = "e"
         return [t1, t2, t3, t4]