python3Packges.websocket: add missing six dependency

```
       > Processing ./websocket_client-0.56.0-py2.py3-none-any.whl
       > ERROR: Could not find a version that satisfies the requirement six (from websocket-client) (from versions: none)
       > ERROR: No matching distribution found for six
       >
```

pkgs/development/python-modules/websocket-client/default.nix

@@ -4,6 +4,7 @@
 , pythonOlder
 , pytestCheckHook
 , python-socks
+, six
 }:
 
 buildPythonPackage rec {
@@ -18,6 +19,7 @@ buildPythonPackage rec {
 
   propagatedBuildInputs = [
     python-socks
+    six
    ];
 
   checkInputs = [