aggressive caching test failure on Windows

[jayvdb]

https://ci.appveyor.com/project/jayvdb/dns-cache/builds/35321024/job/0msnu5mtn32u434c#L102

The hit which should have been cached in the upstream test method was not found/used in test_aggressive.py query, resulting in a real dns query. This is the first time it has occurred, so it is likely a very rare occasional, and the failure result is not critical unless the DNS caching was done because the DNS is no longer available.

self = <tests.test_aggressive.TestCache testMethod=test_hit_additional__<'208.98.188.82'>>
nameserver = '208.98.188.82'
    @foreach(get_nameservers())
    def test_hit_additional(self, nameserver=None):
        if not nameserver:
            raise unittest.SkipTest("unittest_expander leftover")
    
>       self._test_hit_additional(nameserver)
tests\test_upstream.py:612: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
tests\test_aggressive.py:77: in _test_hit_additional
    answer = resolver.query(additional_a_names[0], A)
dns_cache\resolver.py:57: in query
    qname, rdtype=rdtype, rdclass=rdclass, **kwargs
.tox\py36-noapsw\lib\site-packages\dns\resolver.py:1091: in query
    True)
dns_cache\resolver.py:36: in resolve
    raise_on_no_answer, source_port, lifetime,
.tox\py36-noapsw\lib\site-packages\dns\resolver.py:1067: in resolve
    (_, done) = resolution.query_result(None, ex)
dns_cache\block.py:22: in _socket_factory_exception_handler
    raise ex
.tox\py36-noapsw\lib\site-packages\dns\resolver.py:1059: in resolve
    raise_on_truncation=True)
.tox\py36-noapsw\lib\site-packages\dns\query.py:526: in udp
    s = stack.enter_context(_make_socket(af, socket.SOCK_DGRAM, source))
.tox\py36-noapsw\lib\site-packages\dns\query.py:263: in _make_socket
    s = socket_factory(af, type)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
args = (<AddressFamily.AF_INET: 2>, <SocketKind.SOCK_DGRAM: 2>), kwargs = {}
    def _socket_factory_blocker(*args, **kwargs):
>       raise _SocketBlockedError()
E       dns_cache.block._SocketBlockedError: _socket_factory_blocker invoked
dns_cache\block.py:18: _SocketBlockedError