Fix breaking tests on OS X (#43)

OS X resolves "localhost" to loopback ipv4 and ipv6 addresses,
whereas Ubuntu resolves it to just ipv4. This was causing tests
that only expect one listener to fail. The fix is to use
127.0.0.1 instead.

tests/test_connection.py

@@ -10,7 +10,7 @@
 import trustme
 
 
-HOST = 'localhost'
+HOST = '127.0.0.1'
 RESOURCE = '/resource'
 
 @pytest.fixture