[3.12] gh-105497: [Enum] Fix flag mask inversion when unnamed flags exist (GH-106468) (#106620)

gh-105497: [Enum] Fix flag mask inversion when unnamed flags exist (GH-106468)

For example:

    class Flag(enum.Flag):
        A = 0x01
        B = 0x02
        MASK = 0xff

    ~Flag.MASK is Flag(0)
(cherry picked from commit 95b7426f45edb570869a5513c142f29ed9f851a1)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>