input: Write a python function to check if a given sentence is a pangram.
  assert is_pangram("The quick brown fox jumps over the lazy dog") == True
  assert is_pangram("Pack my box with five dozen liquor jugs") == True
  assert is_pangram("Hello world") == False
output: def is_pangram(sentence):
      alphabet = set('abcdefghijklmnopqrstuvwxyz')
      return alphabet <= set(sentence.lower())