tests: Use explicit imports

test/functional/mempool_persist.py

@@ -35,11 +35,12 @@ Test is as follows:
     node1 can't write to disk.
 
 """
+from decimal import Decimal
 import os
 import time
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.util import *
+from test_framework.util import assert_equal, assert_raises_rpc_error, wait_until
 
 class MempoolPersistTest(BitcoinTestFramework):
     def set_test_params(self):