minor: fix flake8 issues

Author: mrava87

pylops/signalprocessing/fft.py

@@ -2,7 +2,7 @@
 
 import logging
 import warnings
-from typing import Literal, Optional, Union
+from typing import Optional, Union
 
 import numpy as np
 import scipy.fft

pylops/signalprocessing/shift.py

@@ -1,6 +1,6 @@
 __all__ = ["Shift"]
 
-from typing import TYPE_CHECKING, Literal, Tuple, Union
+from typing import TYPE_CHECKING, Tuple, Union
 
 import numpy as np