PYTHON-2667 Fix SRV support when running with eventlet

[ShaneHarvey]

This change works around the following eventlet bug:

$ python green_framework_test.py eventlet -s test.test_srv_polling.TestSrvPolling.test_addition
...
test_addition (test.test_srv_polling.TestSrvPolling) ... ERROR

======================================================================
ERROR: test_addition (test.test_srv_polling.TestSrvPolling)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/shane/pymongo-pycharm-3.8/lib/python3.8/site-packages/dns/resolver.py", line 212, in __init__
    rrset = response.find_rrset(response.answer, qname,
  File "/Users/shane/pymongo-pycharm-3.8/lib/python3.8/site-packages/dns/message.py", line 341, in find_rrset
    raise KeyError
KeyError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/shane/pymongo-pycharm-3.8/lib/python3.8/site-packages/dns/resolver.py", line 220, in __init__
    crrset = response.find_rrset(response.answer,
  File "/Users/shane/pymongo-pycharm-3.8/lib/python3.8/site-packages/dns/message.py", line 341, in find_rrset
    raise KeyError
KeyError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/shane/git/mongo-python-driver/pymongo/srv_resolver.py", line 58, in get_options
    results = _resolve(self.__fqdn, 'TXT',
  File "/Users/shane/pymongo-pycharm-3.8/lib/python3.8/site-packages/dns/resolver.py", line 1100, in query
    return get_default_resolver().query(qname, rdtype, rdclass, tcp, source,
  File "/Users/shane/pymongo-pycharm-3.8/lib/python3.8/site-packages/dns/resolver.py", line 1003, in query
    answer = Answer(_qname, rdtype, rdclass, response,
  File "/Users/shane/pymongo-pycharm-3.8/lib/python3.8/site-packages/dns/resolver.py", line 232, in __init__
    raise NoAnswer(response=response)
dns.resolver.NoAnswer: The DNS response does not contain an answer to the question: test1.test.build.10gen.cc. IN TXT

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/shane/git/mongo-python-driver/test/test_srv_polling.py", line 165, in test_addition
    self.run_scenario(response, True)
  File "/Users/shane/git/mongo-python-driver/test/test_srv_polling.py", line 153, in run_scenario
    client = MongoClient(self.CONNECTION_STRING)
  File "/Users/shane/git/mongo-python-driver/pymongo/mongo_client.py", line 659, in __init__
    res = uri_parser.parse_uri(
  File "/Users/shane/git/mongo-python-driver/pymongo/uri_parser.py", line 497, in parse_uri
    dns_options = dns_resolver.get_options()
  File "/Users/shane/git/mongo-python-driver/pymongo/srv_resolver.py", line 64, in get_options
    raise ConfigurationError(str(exc))
pymongo.errors.ConfigurationError: The DNS response does not contain an answer to the question: test1.test.build.10gen.cc. IN TXT

[ShaneHarvey]

I've reported the underlying issue to eventlet: eventlet/eventlet#701