input: Write a python function to fetch popular movies from themoviedb API.
  assert fetch_popular_movies('YOUR_API_KEY') is not None
  assert 'results' in fetch_popular_movies('YOUR_API_KEY')
output: def fetch_popular_movies(api_key):
      import requests
      url = f"https://api.themoviedb.org/3/movie/popular?api_key={api_key}"
      response = requests.get(url)
      if response.status_code == 200:
          return response.json()
      else:
          return None