input: Write a python function to retrieve a value from a lookup table given a key.
  assert lookup_value("key1") == "value1"
  assert lookup_value("key2") == "value2"
  assert lookup_value("key4") == None
output: def lookup_value(key):
      lookup_table = {
          "key1": "value1",
          "key2": "value2",
          "key3": "value3"
      }
      return lookup_table.get(key)