Update mychain.py

mychain.py

@@ -48,6 +48,7 @@ class Blockchain:
             json_object = json.dumps(self.chain, indent=4)
             with open("tmp.json", "w") as outfile:
                 outfile.write(json_object)
+            outfile.close()
             try: 
                 api.upload_file(
                 path_or_fileobj="tmp.json",