input: Write a python function to check if an email is spam based on specific keywords.
  assert is_spam("Get a discount on your next purchase now!") == 'spam'
  assert is_spam("Hello, I hope you are doing well.") == 'not spam'
  assert is_spam("Free offer just for you, purchase now!") == 'spam'
  assert is_spam("This is a regular email.") == 'not spam'
output: def is_spam(email):
      spam_keywords = ["discount", "offer", "free", "purchase", "now"]
      email_lower = email.lower()
      for keyword in spam_keywords:
          if keyword in email_lower:
              return 'spam'
      return 'not spam'