The easiest way to do it is the most risky. It involves dynamic execution and could result in the execution of rogue code so you would have to trust that your input strings are safe. Using your example of
s = "[[['delivery_time', 'waiting_time'], ['short']], [['delivery'], ['fast']], [['delivery'], ['really'], ['great']]]"
you can convert it to a list by
exec("lst = " + s)
to flatten that to a three element (in this example) you could do
lst3 = []
for item in lst:
lst3.append(item)