input: Write a python function to create a simple HTTP server that responds with the current datetime.
  assert 'Server running at localhost:8888' in start_server()
  assert '<p>The time is:' in get_response_from_server()
output: from http.server import BaseHTTPRequestHandler, HTTPServer
  from datetime import datetime
  
  class SimpleHTTPRequestHandler(BaseHTTPRequestHandler):
      def do_GET(self):
          self.send_response(200)
          self.send_header('Content-type', 'text/html')
          self.end_headers()
          current_time = datetime.now().strftime('%Y-%m-%d %H:%M:%S')
          response = f'<html><body><p>The time is: {current_time}</p></body></html>'
          self.wfile.write(response.encode('utf-8'))
  
  def start_server():
      server_address = ('', 8888)
      httpd = HTTPServer(server_address, SimpleHTTPRequestHandler)
      print('Server running at localhost:8888')
      httpd.serve_forever()