code: def _normalize_unit(cls, unit):
          
          if unit in cls.UNITS_IN_SECONDS:
              return unit
          return cls.UNIT_ALIASES_REVERSE.get(unit, None)
docs: Resolve a unit to its real name if it's an alias.
  
          :param unit str: the unit to normalize
          :return: the normalized unit, or None one isn't found
          :rtype: Union[None, str]