input: Write a python function to generate a random string of given length using the characters from 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'.
  assert len(gen_random_str(10)) == 10
  assert len(gen_random_str(15)) == 15
  assert len(gen_random_str(0)) == 0
output: import random
  import string
  
  def gen_random_str(length):
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(length))