Fix return type of KWallet get_credential()

Docs:
get_credential(service, username): Return a credential object stored in
the active keyring. This object contains at least username and password
attributes for the specified service, where the returned username may be
different from the argument.

Prior to this commit, get_credential() returned the password as a string,
without wrapping the credential into a SimpleCredential object.

keyring/backends/kwallet.py

@@ -123,7 +123,8 @@ def get_credential(self, service, username):
         Otherwise, it will return the first username and password combo that it finds.
         """
         if username is not None:
-            return self.get_password(service, username)
+            return super().get_credential(service, username)
+
         if not self.connected(service):
             # the user pressed "cancel" when prompted to unlock their keyring.
             raise KeyringLocked("Failed to unlock the keyring!")
@@ -134,6 +135,8 @@ def get_credential(self, service, username):
             )
             return SimpleCredential(str(username), str(password))
 
+        return None
+
     def set_password(self, service, username, password):
         """Set password for the username of the service"""
         if not self.connected(service):