input: Write a python function to classify an email as 'spam' or 'not spam' based on certain spam words.
  assert classify_email("Congratulations, you win a free vacation!") == 'spam'
  assert classify_email("Please find attached the report for this month.") == 'not spam'
  assert classify_email("Get a full refund on your purchase now!") == 'spam'
  assert classify_email("Here are the meeting notes from our last call.") == 'not spam'
output: def classify_email(email):
      spam_words = ["win", "free", "vacation", "refund", "purchase", "now"]
      email_lower = email.lower()
      for word in spam_words:
          if word in email_lower:
              return 'spam'
      return 'not spam'