input: Write a python function to get zip codes for a given city using a web API.
  assert get_zipcodes_for_city('New York') == {'zip_codes': [10001, 10002, 10003]}
  assert get_zipcodes_for_city('San Francisco') == {'zip_codes': [94102, 94103, 94104]}
  assert get_zipcodes_for_city('Los Angeles') == {'zip_codes': [90001, 90002, 90003]}
output: import requests
  
  def get_zipcodes_for_city(city):
      api_url = f"http://api.zippopotam.us/us/{city.replace(' ', '%20')}"
      response = requests.get(api_url)
      if response.status_code == 200:
          data = response.json()
          zip_codes = [place['post code'] for place in data['places']]
          return {'zip_codes': zip_codes}
      else:
          return {'zip_codes': []}