input: Write a python function to extract the skills from the provided HTML string.
  assert extract_skills(html_str) == ['Python', 'SQL', 'JavaScript', 'HTML/CSS']
  assert extract_skills(html_str_with_no_skills) == []
output: def extract_skills(html_str):
      from bs4 import BeautifulSoup
  
      soup = BeautifulSoup(html_str, 'html.parser')
      skills = [li.text for li in soup.find_all('li', class_='skill')]
      return skills