input: Write a python function to check if a sentence is a pangram.
  assert is_pangram("The quick brown fox jumps over the lazy dog") == True
  assert is_pangram("Hello World") == False
  assert is_pangram("Sphinx of black quartz, judge my vow") == True
output: def is_pangram(sentence):
      alphabet = set('abcdefghijklmnopqrstuvwxyz')
      return alphabet <= set(sentence.lower())