input: Write a python function to listen for a command using the speech_recognition library.
  assert listen_command() == 'Hello'
  assert listen_command() == 'Please turn on the lights'
  assert listen_command() == 'What time is it?'
output: def listen_command():
      import speech_recognition as sr
      
      recognizer = sr.Recognizer()
      with sr.Microphone() as source:
          print("Listening for a command...")
          audio = recognizer.listen(source)
          try:
              command = recognizer.recognize_google(audio)
              return command
          except sr.UnknownValueError:
              return "Could not understand the audio"
          except sr.RequestError:
              return "Could not request results; check your network connection"