TypeError: must be str, not bytes

import time
import hmac
import urllib
import requests
import hashlib
import base64
import sys
import json
 
API_KEY = 'ac85...4173abdf9...8e7d2202'
API_SECRET = 'KY9R6...QeDr2kVrq...oGk0Ev36Z...uafbsk'
 
 
def api_query( method, req = None ):
 if not req:
         req = {}
 print ("def api_query( method = " + method + ", req = " + str( req ) + " ):")
 time.sleep( 1 )
 public_set = set([ "GetCurrencies", "GetTradePairs", "GetMarkets", "GetMarket", "GetMarketHistory", "GetMarketOrders" ])
 private_set = set([ "GetBalance", "GetDepositAddress", "GetOpenOrders", "GetTradeHistory", "GetTransactions", "SubmitTrade", "CancelTrade", "SubmitTip" ])
 if method in public_set:
         url = "https://www.cryptopia.co.nz/api/" + method
         if req:
             for param in req:
                 url += '/' + str( param )
         r = requests.get( url )
 elif method in private_set:
         url = "https://www.cryptopia.co.nz/Api/" + method
         nonce = str( int( time.time() ) )
         post_data = json.dumps( req );
         m = hashlib.md5()
         m.update(post_data.encode('utf-8'))
         requestContentBase64String = base64.b64encode(m.digest())
         signature = API_KEY + "POST" + urllib.parse.quote_plus( url ).lower() + nonce + requestContentBase64String
         hmacsignature = base64.b64encode(hmac.new(base64.b64decode( API_SECRET ), signature, hashlib.sha256).digest())
         header_value = "amx " + API_KEY + ":" + hmacsignature + ":" + nonce
         headers = { 'Authorization': header_value, 'Content-Type':'application/json; charset=utf-8' }
         r = requests.post( url, data = post_data, headers = headers )
 response = r.text
 print ("( Response ): " + response)
 return response.replace("false","False").replace("true","True").replace('":null','":None' )
 
 
# Public:
# +
# print api_query("GetCurrencies")
 
# +
#print (api_query("GetMarket", [ 101, 6 ] ))
# {"Success":True,"Message":None,"Data":{"TradePairId":100,"Label":"DOT/BTC","AskPrice":0.00000020,"BidPrice":0.00000019,"Low":0.00000019,"High":0.00000021,"Volume":1263556.65136394,"LastPrice":0.00000019,"LastVolume":774.83684404,"BuyVolume":50896673.08961847,"SellVolume":33046510.52562918,"Change":0.0},"Error":None}
 
# Private:
print (api_query("GetBalance"))
 
# +
# print api_query("GetBalance", {'CurrencyId':2} )