input: Write a python function to format a phone number.
  assert format_phone_number("1234567890") == "(123) 456-7890"
  assert format_phone_number("0987654321") == "(098) 765-4321"
  assert format_phone_number("5556667777") == "(555) 666-7777"
output: def format_phone_number(number):
      return f"({number[:3]}) {number[3:6]}-{number[6:]}"