input: Write a python function to send an HTML file to a web server using a POST request and return the status code.
  assert send_html_file('http://example.com', 'index.html') == 200
  assert send_html_file('http://example.com', 'nonexistent.html') == 404
  assert send_html_file('http://invalid.url', 'index.html') == None
output: def send_html_file(url, filepath):
      import requests
      try:
          with open(filepath, 'r') as file:
              html_content = file.read()
          response = requests.post(url, data={'html': html_content})
          return response.status_code
      except FileNotFoundError:
          return 404
      except requests.exceptions.RequestException:
          return None