Use `==` to compare against the empty string, not `is`

py38 will start complaining like

   SyntaxWarning: "is" with a literal. Did you mean "=="?

Change-Id: I799e288f66230a7847a13b7f303e4cd8e63eea59

swift/common/daemon.py

@@ -267,7 +267,7 @@ def run_daemon(klass, conf_file, section_name='', once=False, **kwargs):
     """
     # very often the config section_name is based on the class name
     # the None singleton will be passed through to readconf as is
-    if section_name is '':
+    if section_name == '':
         section_name = sub(r'([a-z])([A-Z])', r'\1-\2',
                            klass.__name__).lower()
     try:

swift/common/middleware/acl.py

@@ -217,7 +217,7 @@ def parse_acl_v2(data):
     """
     if data is None:
         return None
-    if data is '':
+    if data == '':
         return {}
     try:
         result = json.loads(data)

test/unit/common/middleware/s3api/test_s3request.py

@@ -187,7 +187,7 @@ class TestRequest(S3ApiTestCase):
         self.assertEqual(mock_get_resp.call_count, 2)
         args, kargs = mock_get_resp.call_args_list[0]
         get_resp_obj = args[3]
-        self.assertTrue(get_resp_obj is '')
+        self.assertEqual(get_resp_obj, '')
         self.assertEqual(m_check_permission.call_count, 1)
         args, kargs = m_check_permission.call_args
         permission = args[1]

test/unit/common/test_wsgi.py

@@ -600,7 +600,7 @@ class TestWSGI(unittest.TestCase):
                 logger = logging.getLogger('test')
                 sock = listen_zero()
                 wsgi.run_server(conf, logger, sock)
-                self.assertTrue(os.environ['TZ'] is not '')
+                self.assertNotEqual(os.environ['TZ'], '')
 
         self.assertEqual(30, _wsgi.WRITE_TIMEOUT)
         _wsgi_evt.hubs.use_hub.assert_called_with(utils.get_hub())