kholidays: update to 5.61.0

5.61.0
  Add public holidays' substitute days in Russia, for 2019-2020
  Update holidays in Russia
5.60.0
  Update UK Early May bank holiday for 2020
  Fix ISO code for Hesse / Germany
5.58.0
  holidays/plan2/holiday_zm_en-gb - add Zambia holidays
  holidays/plan2/holiday_lv_lv - fix Midsummer's Day
  holiday_mu_en - Holidays 2019 in Mauritius
  holiday_th_en-gb - update for 2019
  Update Japanese holidays
  Add public holidays for Lower Saxony (Germany)

(markd)