devtools: make github-merge.py use py3

This makes github-merge.py the first developer tool to go
all Python 3 (for context see #7717).

The changes are straightforward as the script already was
`from __future__ import division,print_function,unicode_literals`.

However urllib2 changed name, and json will only accept unicode data not
bytes.

This retains py2 compatibility for now: not strictly necessary
as it's not used by the build system - but it was easy.

contrib/devtools/github-merge.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 # Copyright (c) 2016 Bitcoin Core Developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
@@ -19,6 +19,11 @@ import os,sys
 from sys import stdin,stdout,stderr
 import argparse
 import subprocess
+import json,codecs
+try:
+    from urllib.request import Request,urlopen
+except:
+    from urllib2 import Request,urlopen
 
 # External tools (can be overridden using environment)
 GIT = os.getenv('GIT','git')
@@ -38,7 +43,7 @@ def git_config_get(option, default=None):
     Get named configuration option from git repository.
     '''
     try:
-        return subprocess.check_output([GIT,'config','--get',option]).rstrip()
+        return subprocess.check_output([GIT,'config','--get',option]).rstrip().decode('utf-8')
     except subprocess.CalledProcessError as e:
         return default
 
@@ -47,18 +52,19 @@ def retrieve_pr_title(repo,pull):
     Retrieve pull request title from github.
     Return None if no title can be found, or an error happens.
     '''
-    import urllib2,json
     try:
-        req = urllib2.Request("https://api.github.com/repos/"+repo+"/pulls/"+pull)
-        result = urllib2.urlopen(req)
-        result = json.load(result)
-        return result['title']
+        req = Request("https://api.github.com/repos/"+repo+"/pulls/"+pull)
+        result = urlopen(req)
+        reader = codecs.getreader('utf-8')
+        obj = json.load(reader(result))
+        return obj['title']
     except Exception as e:
         print('Warning: unable to retrieve pull title from github: %s' % e)
         return None
 
 def ask_prompt(text):
     print(text,end=" ",file=stderr)
+    stderr.flush()
     reply = stdin.readline().rstrip()
     print("",file=stderr)
     return reply