input: Write a python function to get the median income from World Bank API and return it as a dictionary.
  assert get_median_income() == {'2015': 15.55, '2016': 15.50, '2017': 15.45, '2018': 15.40}
  assert '2020' in get_median_income().keys()
  assert isinstance(get_median_income(), dict)
output: import requests
  
  def get_median_income():
      url = "https://api.worldbank.org/v2/country/USA/indicator/SI.POV.DDAY?format=json"
      response = requests.get(url)
      data = response.json()
  
      median_income = {}
      for entry in data[1]:
          year = entry['date']
          value = entry['value']
          if value is not None:
              median_income[year] = value
  
      return median_income