Get age the game was won in.

def _won_in(self):
        """Get age the game was won in."""
        if not self._summary['finished']:
            return
        starting_age = self._summary['settings']['starting_age'].lower()
        if starting_age == 'post imperial':
            starting_age = 'imperial'
        ages_reached = set([starting_age])
        for player in self._summary['players']:
            for age, reached in player['ages'].items():
                if reached:
                    ages_reached.add(age)
        ages = ['imperial', 'castle', 'feudal', 'dark']
        for age in ages:
            if age in ages_reached:
                return age