fix byte/str type mismatch

Author: NIC619

ethereum/pow/chain.py

@@ -365,7 +365,7 @@ def add_block(self, block):
         self.add_child(block)
         self.db.put('head_hash', self.head_hash)
         self.db.put(block.hash, rlp.encode(block))
-        self.db.put(b'changed:'+block.hash, b''.join(list(changed.keys())))
+        self.db.put(b'changed:'+block.hash, b''.join([k.encode() for k in list(changed.keys())]))
         print('Saved %d address change logs' % len(changed.keys()))
         self.db.put(b'deletes:'+block.hash, b''.join(deletes))
         print('Saved %d trie node deletes for block %d (%s)' % (len(deletes), block.number, utils.encode_hex(block.hash)))