input: Write a Python function to replace email addresses in a given string with "<valid email>" using regular expressions.
  assert replace_emails("Contact us at support@example.com for assistance.") == "Contact us at <valid email> for assistance."
  assert replace_emails("Send your feedback to feedback@example.com or admin@example.org.") == "Send your feedback to <valid email> or <valid email>."
  assert replace_emails("my_email@example.com is my email address.") == "<valid email> is my email address."
output: def replace_emails(text):
      import re
      return re.sub(r'\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}\b', '<valid email>', text)