test_urllib2_localnet: clear server variable

[vstinner]

Set the server attribute to None in cleanup to avoid dangling
threads.

[vstinner]

This change should fix these warnings:

http://buildbot.python.org/all/builders/AMD64%20Windows8.1%20Refleaks%203.x/builds/38/steps/test/logs/stdio

0:33:30 [160/406] test_urllib2_localnet failed (env changed) (221 sec) -- running: test_decimal (267 sec), test_logging (119 sec), test_io (96 sec)
Warning -- threading_cleanup() failed to cleanup 0 threads after 11 sec (count: 0, dangling: 18)
beginning 6 repetitions
123456
Warning -- threading_cleanup() failed to cleanup 0 threads after 11 sec (count: 0, dangling: 18)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 10 sec (count: 0, dangling: 18)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 10 sec (count: 0, dangling: 18)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 10 sec (count: 0, dangling: 18)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 10 sec (count: 0, dangling: 18)
.Warning -- threading_cleanup() failed to cleanup 0 threads after 10 sec (count: 0, dangling: 18)
.