Enable W191 and W291 flake8 checks.

Remove trailing whitespace from Python files.
Convert tabs to spaces.

test/functional/feature_maxuploadtarget.py

@@ -100,7 +100,7 @@ class MaxUploadTest(BitcoinTestFramework):
             assert_equal(p2p_conns[0].block_receive_map[big_old_block], i+1)
 
         assert_equal(len(self.nodes[0].getpeerinfo()), 3)
-        # At most a couple more tries should succeed (depending on how long 
+        # At most a couple more tries should succeed (depending on how long
         # the test has been running so far).
         for i in range(3):
             p2p_conns[0].send_message(getdata_request)