Partial Merge #11835: Add Travis check for unused Python imports

d60b32074 Add Travis check for unused Python imports (practicalswift)
c7399e708 Remove unused Python imports (practicalswift)

Pull request description:

  Add Travis check for unused Python imports.

  ```
  $ contrib/devtools/lint-python.sh
  ./test/functional/example_test.py:18:1: F401 'test_framework.mininode.NODE_NETWORK' imported but unused
  ./test/functional/test_framework/messages.py:27:1: F401 'test_framework.util.wait_until' imported but unused
  ./test/functional/test_framework/test_framework.py:16:1: F401 'traceback' imported but unused
  ```

Tree-SHA512: 78e50fb1488abe3ebe365e766cb8d6d448cf1bd16c8691e102cb9bf7c202988bdf6e10b25ff772c62e05c72568168462e88cdc7ad98069d9eb3be727735b2d56

contrib/devtools/lint-python.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+#
+# Copyright (c) 2017 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# Check for specified flake8 warnings in python files.
+
+# F401: module imported but unused
+flake8 --ignore=B,C,E,F,I,N,W --select=F401 .

test/functional/example_test.py

@@ -21,7 +21,6 @@ from test_framework.mininode import (
     mininode_lock,
     msg_block,
     msg_getdata,
-    NODE_NETWORK,
     network_thread_join,
     network_thread_start,
 )

test/functional/test_framework/messages.py

@@ -22,7 +22,7 @@ import struct
 import time
 
 from test_framework.siphash import siphash256
-from test_framework.util import hex_str_to_bytes, bytes_to_hex_str, wait_until
+from test_framework.util import hex_str_to_bytes, bytes_to_hex_str
 
 import dash_hash
 

test/functional/test_framework/test_framework.py

@@ -15,7 +15,6 @@ import shutil
 import sys
 import tempfile
 import time
-import traceback
 from concurrent.futures import ThreadPoolExecutor
 
 from .authproxy import JSONRPCException