input: Write a python function to build a dictionary with city information.
  assert build_city_info_dictionary('Dallas', 'Texas, USA', '1.3 million') == {'Dallas': ('Texas, USA', '1.3 million')}
  assert build_city_info_dictionary('New York', 'New York, USA', '8.4 million') == {'New York': ('New York, USA', '8.4 million')}
  assert build_city_info_dictionary('Tokyo', 'Kanto, Japan', '37 million') == {'Tokyo': ('Kanto, Japan', '37 million')}
output: def build_city_info_dictionary(city, region_country, population):
      return {city: (region_country, population)}