fix: type hint update for python3.9

Author: changsookim

src/qibo/backends/abstract.py

@@ -31,15 +31,15 @@ def __repr__(self):
 
     @property
     @abc.abstractmethod
-    def qubits(self) -> Optional[list[int | str]]:  # pragma: no cover
+    def qubits(self) -> Optional[list[Union[int, str]]]:  # pragma: no cover
         """Return the qubit names of the backend. If :class:`SimulationBackend`, return None."""
         raise_error(NotImplementedError)
 
     @property
     @abc.abstractmethod
     def connectivity(
         self,
-    ) -> Optional[list[tuple[int | str, int | str]]]:  # pragma: no cover
+    ) -> Optional[list[tuple[Union[int, str], Union[int, str]]]]:  # pragma: no cover
         """Return the available qubit pairs of the backend. If :class:`SimulationBackend`, return None."""
         raise_error(NotImplementedError)