input: Write a python function to parse an HTML document and return the title of the document.
  assert HTMLParser("<html><head><title>Test Title</title></head><body></body></html>").parse().title.string == "Test Title"
  assert HTMLParser("<html><head><title>Another Title</title></head><body></body></html>").parse().title.string == "Another Title"
  assert HTMLParser("<html><head><title>Sample Title</title></head><body></body></html>").parse().title.string == "Sample Title"
output: from bs4 import BeautifulSoup
  
  class HTMLParser:
      def __init__(self, html):
          self.html = html
      
      def parse(self):
          return BeautifulSoup(self.html, 'html.parser')