input: Write a python function to convert a Unix timestamp to a human-readable date format.
  assert convert_unix_to_human(1545732721) == '2018-12-25 02:32:01'
  assert convert_unix_to_human(1609459200) == '2021-01-01 00:00:00'
  assert convert_unix_to_human(0) == '1970-01-01 00:00:00'
output: def convert_unix_to_human(unix_timestamp):
      from datetime import datetime
      return datetime.utcfromtimestamp(unix_timestamp).strftime('%Y-%m-%d %H:%M:%S')