input: Write a function to check if a password is valid with at least 8 characters, one uppercase letter, one lowercase letter, and one digit.
  assert is_valid_password('Password123')==True
  assert is_valid_password('password123')==False
  assert is_valid_password('PASSWORD123')==False
  assert is_valid_password('Passwrd')==False
output: def is_valid_password(password):
      if len(password) < 8:
          return False
      has_upper = any(c.isupper() for c in password)
      has_lower = any(c.islower() for c in password)
      has_digit = any(c.isdigit() for c in password)
      return has_upper and has_lower and has_digit