input: Write a function to check if a given phone number is valid.
  assert valid_phone_number("1234567890") == True
  assert valid_phone_number("0987654321") == True
  assert valid_phone_number("12345") == False
  assert valid_phone_number("12345678901") == False
  assert valid_phone_number("123456789a") == False
output: def valid_phone_number(phone_number):
      return phone_number.isdigit() and len(phone_number) == 10