ignore that cache decorator does not exisit in python 3.8

eos/instrument.py

@@ -3,12 +3,17 @@ Classes describing the AMOR instrument configuration used during reduction.
 """
 
 import logging
-from functools import cache
-
 import numpy as np
 
 from . import const
 
+try:
+    from functools import cache
+except ImportError:
+    # python <3.9
+    def cache(func): return func
+
+
 class Detector:
     nBlades  = 14  # number of active blades in the detector
     nWires   = 32  # number of wires per blade