PYTHON-2667 Fix SRV support when running with eventlet (#612)

(cherry picked from commit acfa7b6)

Author: ShaneHarvey

pymongo/srv_resolver.py

@@ -16,13 +16,6 @@
 
 try:
     from dns import resolver
-
-    try:
-        # dnspython >= 2
-        from dns.resolver import resolve as _resolve
-    except ImportError:
-        # dnspython 1.X
-        from dns.resolver import query as _resolve
     _HAVE_DNSPYTHON = True
 except ImportError:
     _HAVE_DNSPYTHON = False
@@ -45,6 +38,15 @@ def maybe_decode(text):
         return text
 
 
+# PYTHON-2667 Lazily call dns.resolver methods for compatibility with eventlet.
+def _resolve(*args, **kwargs):
+    if hasattr(resolver, 'resolve'):
+        # dnspython >= 2
+        return resolver.resolve(*args, **kwargs)
+    # dnspython 1.X
+    return resolver.query(*args, **kwargs)
+
+
 class _SrvResolver(object):
     def __init__(self, fqdn, connect_timeout=None):
         self.__fqdn = fqdn