Enable W191 and W291 flake8 checks.

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

test/functional/p2p_segwit.py

@@ -1659,7 +1659,7 @@ class SegWitTest(BitcoinTestFramework):
             tx2.wit.vtxinwit.append(CTxInWitness())
             tx2.wit.vtxinwit[-1].scriptWitness.stack = [ witness_program ]
             total_value += tx.vout[i].nValue
-        tx2.wit.vtxinwit[-1].scriptWitness.stack = [ witness_program_toomany ] 
+        tx2.wit.vtxinwit[-1].scriptWitness.stack = [ witness_program_toomany ]
         tx2.vout.append(CTxOut(total_value, CScript([OP_TRUE])))
         tx2.rehash()