input: Write a python function to check if a given number is even or odd.
  assert check_even_odd(2)=="2 is an even number."
  assert check_even_odd(3)=="3 is an odd number."
  assert check_even_odd(0)=="0 is an even number."
  assert check_even_odd(11)=="11 is an odd number."
output: def check_even_odd(number):
      if number % 2 == 0:
          return f"{number} is an even number."
      else:
          return f"{number} is an odd number."