input: Write a python function to generate all permutations of a given list of letters and return them as a list of strings.
  assert generate_permutations(["a", "b", "t"]) == ['abt', 'atb', 'bat', 'bta', 'tab', 'tba']
  assert generate_permutations(["x", "y"]) == ['xy', 'yx']
  assert generate_permutations(["p"]) == ['p']
output: def generate_permutations(letters):
      from itertools import permutations
      return [''.join(p) for p in permutations(letters)]