input: Write a python function to check if a website is up using the requests module.
  assert check_website('http://example.com') == 'Website is up and running.'
  assert check_website('http://nonexistentwebsite.com') == 'Website is not responding.'
  assert check_website('http://google.com') == 'Website is up and running.'
output: def check_website(url):
      import requests
      try:
          response = requests.get(url)
          if response.status_code == 200:
              return 'Website is up and running.'
          else:
              return 'Website is not responding.'
      except requests.exceptions.RequestException:
          return 'Website is not responding.'