Fix check for signed logout

```
************* Module saml2.client
src/saml2/client.py:225:27: E1101: Instance of 'Saml2Client' has no 'logout_requests_signed' member (no-member)
```

The reference should be through the config member of the Saml2Client object.

Signed-off-by: Ivan Kanakarakis <[email protected]>

Author: c00kiemon5ter

src/saml2/client.py

@@ -222,7 +222,7 @@ def do_logout(self, name_id, entity_ids, reason, expire, sign=None,
                     sign = True
 
                 if sign is None:
-                    sign = self.logout_requests_signed
+                    sign = self.config.logout_requests_signed
 
                 sigalg = None
                 if sign: