Update tree.py (#215)

Line 932 gives this error "UnicodeEncodeError: 'charmap' codec can't encode characters in position 20-22: character maps to <undefined>" , solved it using the encode function.

treelib/tree.py

@@ -929,7 +929,7 @@ def write(line):
             print("Tree is empty")
 
         if stdout:
-            print(self._reader)
+            print(self._reader.encode('utf-8'))
         else:
             return self._reader