concurrent_download.py: update for py3

examples/concurrent_download.py

@@ -3,6 +3,7 @@
 
 """Spawn multiple workers and wait for them to complete"""
 from __future__ import print_function
+import sys
 
 urls = ['http://www.google.com', 'http://www.yandex.ru', 'http://www.python.org']
 
@@ -12,12 +13,16 @@ from gevent import monkey
 # patches stdlib (including socket and ssl modules) to cooperate with other greenlets
 monkey.patch_all()
 
-import urllib2
+
+if sys.version_info[0] == 3:
+    from urllib.request import urlopen
+else:
+    from urllib2 import urlopen
 
 
 def print_head(url):
     print('Starting %s' % url)
-    data = urllib2.urlopen(url).read()
+    data = urlopen(url).read()
     print('%s: %s bytes: %r' % (url, len(data), data[:50]))
 
 jobs = [gevent.spawn(print_head, url) for url in urls]