input: Write a function to generate a random string of specified length using letters from ascii_letters.
  assert len(generate_string(5)) == 5
  assert len(generate_string(10)) == 10
  assert all(c in string.ascii_letters for c in generate_string(15))
output: import string
  import random
  
  def generate_string(length):
      return ''.join(random.choice(string.ascii_letters) for _ in range(length))