Returns a list of usernames of approvers who approved this approval.

def GetNonExpiredApprovers(self):
    """Returns a list of usernames of approvers who approved this approval."""

    lifetime = rdfvalue.Duration(
        self.Get(self.Schema.LIFETIME) or config.CONFIG["ACL.token_expiry"])

    # Check that there are enough approvers.
    #
    # TODO(user): approvals have to be opened with
    # age=aff4.ALL_TIMES because versioning is used to store lists
    # of approvers. This doesn't seem right and has to be fixed.
    approvers = set()
    now = rdfvalue.RDFDatetime.Now()
    for approver in self.GetValuesForAttribute(self.Schema.APPROVER):
      if approver.age + lifetime > now:
        approvers.add(utils.SmartStr(approver))

    return list(approvers)