dumbhttp: use IPv6 if HGIPV6 is set to 1

This will fix flaky tests using dumbhttp.

The patch was tested on gcc112.fsffrance.org using the following command:

  ./run-tests.py -j 40 --runs-per-test 120 test-bundle2-remote-changegroup.t

tests/dumbhttp.py

@@ -1,55 +1,63 b''
 #!/usr/bin/env python
 
 from __future__ import absolute_import
 
 """
 Small and dumb HTTP server for use in tests.
 """
 
 import optparse
+import os
 import signal
+import socket
 import sys
 
 from mercurial import (
     server,
     util,
 )
 
 httpserver = util.httpserver
 OptionParser = optparse.OptionParser
 
+if os.environ.get('HGIPV6', '0') == '1':
+    class simplehttpserver(httpserver.httpserver):
+        address_family = socket.AF_INET6
+else:
+    simplehttpserver = httpserver.httpserver
+
 class simplehttpservice(object):
     def __init__(self, host, port):
         self.address = (host, port)
     def init(self):
-        self.httpd = httpserver.httpserver(
+        self.httpd = simplehttpserver(
             self.address, httpserver.simplehttprequesthandler)
     def run(self):
         self.httpd.serve_forever()
 
 if __name__ == '__main__':
     parser = OptionParser()
     parser.add_option('-p', '--port', dest='port', type='int', default=8000,
         help='TCP port to listen on', metavar='PORT')
     parser.add_option('-H', '--host', dest='host', default='localhost',
         help='hostname or IP to listen on', metavar='HOST')
     parser.add_option('--pid', dest='pid',
         help='file name where the PID of the server is stored')
     parser.add_option('-f', '--foreground', dest='foreground',
         action='store_true',
         help='do not start the HTTP server in the background')
     parser.add_option('--daemon-postexec', action='append')
 
     (options, args) = parser.parse_args()
 
     signal.signal(signal.SIGTERM, lambda x, y: sys.exit(0))
 
     if options.foreground and options.pid:
         parser.error("options --pid and --foreground are mutually exclusive")
 
     opts = {'pid_file': options.pid,
             'daemon': not options.foreground,
             'daemon_postexec': options.daemon_postexec}
     service = simplehttpservice(options.host, options.port)
     server.runservice(opts, initfn=service.init, runfn=service.run,
                       runargs=[sys.executable, __file__] + sys.argv[1:])