Added lru_cache decorator to FormatToken.is_augassign for consistency with FormatToken.is_arithmetic_op

Author: Andrea-Oliveri

yapf/yapflib/format_token.py

@@ -333,6 +333,7 @@ def is_assign(self):
     return subtypes.ASSIGN_OPERATOR in self.subtypes
 
   @property
+  @lru_cache()
   def is_augassign(self):
     return self.value in frozenset({
         '+=',