Remove Python 2.x compatibility from socket client (#801)

* Remove Python 2.x compatibility from socket client

* Update socket_client.py

pychromecast/socket_client.py

@@ -15,7 +15,6 @@
 import select
 import socket
 import ssl
-import sys
 import threading
 import time
 from collections import defaultdict
@@ -106,17 +105,9 @@ def _message_to_string(message, data=None):
     )
 
 
-if sys.version_info >= (3, 0):
-
-    def _json_to_payload(data):
-        """Encodes a python value into JSON format."""
-        return json.dumps(data, ensure_ascii=False).encode("utf8")
-
-else:
-
-    def _json_to_payload(data):
-        """Encodes a python value into JSON format."""
-        return json.dumps(data, ensure_ascii=False)
+def _json_to_payload(data: dict) -> bytes:
+    """Encodes a python value into JSON format."""
+    return json.dumps(data, ensure_ascii=False).encode("utf8")
 
 
 @dataclass(frozen=True)