Begin working on gateway v4 support.

Bump websockets requirement to v3.1

Should be squashed...

discord/errors.py

@@ -101,3 +101,21 @@ class LoginFailure(ClientException):
     failure.
     """
     pass
+
+class ConnectionClosed(ClientException):
+    """Exception that's thrown when the gateway connection is
+    closed for reasons that could not be handled internally.
+
+    Attributes
+    -----------
+    code : int
+        The close code of the websocket.
+    reason : str
+        The reason provided for the closure.
+    """
+    def __init__(self, original):
+        # This exception is just the same exception except
+        # reconfigured to subclass ClientException for users
+        self.code = original.code
+        self.reason = original.reason
+        super().__init__(str(original))