[3.12] gh-115539: Allow enum.Flag to have None members (GH-115636) (GH-115694)

gh-115539: Allow enum.Flag to have None members (GH-115636)
(cherry picked from commit c2cb31bbe1262213085c425bc853d6587c66cae9)

Co-authored-by: Jason Zhang <yurenzhang2017@gmail.com>