input: Write a python function to construct a web crawler to retrieve the URL of all the articles posted on a given page.
  assert get_urls('https://www.example.com/test1') == {'https://www.example.com/article1', 'https://www.example.com/article2'}
  assert get_urls('https://www.example.com/test2') == {'https://www.example.com/article3', 'https://www.example.com/article4'}
  assert get_urls('https://www.example.com/test3') == {'https://www.example.com/article5', 'https://www.example.com/article6'}
output: def get_urls(page_url):
      import requests
      from bs4 import BeautifulSoup
  
      response = requests.get(page_url)
      soup = BeautifulSoup(response.text, 'html.parser')
      
      article_urls = set()
      for link in soup.find_all('a', href=True):
          href = link['href']
          if 'article' in href:
              article_urls.add(href if href.startswith('http') else page_url + href)
  
      return article_urls