tests: Use explicit imports

test/functional/p2p_sendheaders.py

@@ -86,9 +86,9 @@ e. Announce one more that doesn't connect.
    Expect: disconnect.
 """
 from test_framework.blocktools import create_block, create_coinbase
+from test_framework.messages import CInv
 from test_framework.mininode import (
     CBlockHeader,
-    CInv,
     NODE_WITNESS,
     P2PInterface,
     mininode_lock,