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

@@ -7,7 +7,9 b' Small and dumb HTTP server for use in te'
 """
 
 import optparse
+import os
 import signal
+import socket
 import sys
 
 from mercurial import (
@@ -18,11 +20,17 b' from mercurial import ('
 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()