fix: clear logger_ from saved state to fix #779

pysr/sr.py

@@ -1225,6 +1225,7 @@ def __getstate__(self) -> dict[str, Any]:
                     f"`{state_key}` at runtime."
                 )
         state_keys_to_clear = state_keys_containing_lambdas
+        state_keys_to_clear.append("logger_")
         pickled_state = {
             key: (None if key in state_keys_to_clear else value)
             for key, value in state.items()