Fix problem from GitHub PR #127

Makefile

 PROJECT = yangson
-VERSION = 1.4.16
+VERSION = 1.4.17
 .PHONY = tags deps install-deps test
 
 tags:

yangson/exceptions.py

@@ -211,7 +211,7 @@ class NotSupported(ParserException):
         self.feature = feature
 
     def __str__(self: "NotSupported") -> str:
-        return super().str() + ": " + str(self.feature)
+        return super().__str__() + ": " + str(self.feature)
 
 
 class MissingModule(YangsonException):