Update seeds tooling to Python 3

All the other tooling scripts require Python 3, it makes sense to do so
here too.

Also document the dependency on python3-dnspython.

contrib/seeds/README.md

@@ -1,4 +1,4 @@
-### Seeds ###
+# Seeds
 
 Utility to generate the seeds.txt list that is compiled into the client
 (see [src/chainparamsseeds.h](/src/chainparamsseeds.h) and other utilities in [contrib/seeds](/contrib/seeds)).
@@ -6,6 +6,11 @@ Utility to generate the seeds.txt list that is compiled into the client
 The seeds compiled into the release are created from sipa's DNS seed data, like this:
 
     curl -s http://bitcoin.sipa.be/seeds.txt > seeds_main.txt
-    python makeseeds.py < seeds_main.txt > nodes_main.txt
-    python generate-seeds.py . > ../../src/chainparamsseeds.h
+    python3 makeseeds.py < seeds_main.txt > nodes_main.txt
+    python3 generate-seeds.py . > ../../src/chainparamsseeds.h
 
+## Dependencies
+
+Ubuntu:
+
+    sudo apt-get install python3-dnspython

contrib/seeds/generate-seeds.py

@@ -1,5 +1,5 @@
-#!/usr/bin/env python
-# Copyright (c) 2014 Wladimir J. van der Laan
+#!/usr/bin/env python3
+# Copyright (c) 2014-2017 Wladimir J. van der Laan
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 '''
@@ -31,7 +31,7 @@ The output will be two data structures with the peers in binary format:
 
 These should be pasted into `src/chainparamsseeds.h`.
 '''
-from __future__ import print_function, division
+
 from base64 import b32decode
 from binascii import a2b_hex
 import sys, os

contrib/seeds/makeseeds.py

@@ -1,5 +1,5 @@
-#!/usr/bin/env python
-# Copyright (c) 2013-2016 The Bitcoin Core developers
+#!/usr/bin/env python3
+# Copyright (c) 2013-2017 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 #
@@ -14,13 +14,13 @@ MIN_BLOCKS = 337600
 
 # These are hosts that have been observed to be behaving strangely (e.g.
 # aggressively connecting to every node).
-SUSPICIOUS_HOSTS = set([
+SUSPICIOUS_HOSTS = {
     "130.211.129.106", "178.63.107.226",
     "83.81.130.26", "88.198.17.7", "148.251.238.178", "176.9.46.6",
     "54.173.72.127", "54.174.10.182", "54.183.64.54", "54.194.231.211",
     "54.66.214.167", "54.66.220.137", "54.67.33.14", "54.77.251.214",
     "54.94.195.96", "54.94.200.247"
-])
+}
 
 import re
 import sys
@@ -104,7 +104,7 @@ def filtermultiport(ips):
     hist = collections.defaultdict(list)
     for ip in ips:
         hist[ip['sortkey']].append(ip)
-    return [value[0] for (key,value) in hist.items() if len(value)==1]
+    return [value[0] for (key,value) in list(hist.items()) if len(value)==1]
 
 # Based on Greg Maxwell's seed_filter.py
 def filterbyasn(ips, max_per_asn, max_total):
@@ -164,9 +164,9 @@ def main():
 
     for ip in ips:
         if ip['net'] == 'ipv6':
-            print '[%s]:%i' % (ip['ip'], ip['port'])
+            print('[%s]:%i' % (ip['ip'], ip['port']))
         else:
-            print '%s:%i' % (ip['ip'], ip['port'])
+            print('%s:%i' % (ip['ip'], ip['port']))
 
 if __name__ == '__main__':
     main()