Made P2WSH_PRE be the correct value

The previous value of '0016' doesnt appear in any literature. According
to BIP141 the options are '0014' for p2wpkh or '0020' for p2wsh

jmbitcoin/jmbitcoin/secp256k1_transaction.py

@@ -17,7 +17,7 @@ from jmbitcoin.bech32 import *
 P2PKH_PRE, P2PKH_POST = b'\x76\xa9\x14', b'\x88\xac'
 P2SH_P2WPKH_PRE, P2SH_P2WPKH_POST = b'\xa9\x14', b'\x87'
 P2WPKH_PRE = b'\x00\x14'
-P2WSH_PRE = b'\x00\x16'
+P2WSH_PRE = b'\x00\x20'
 
 # Transaction serialization and deserialization
 
@@ -884,4 +884,4 @@ def make_shuffled_tx(ins, outs, deser=True, version=1, locktime=0):
     if deser:
         return deserialize(tx)
     else:
-        return tx
+        return tx