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I suppose you'll also forbid adding new members to Flags, just like Enum does, right? (https://docs.python.org/3/library/enum.html#restricted-subclassing-of-enumerations) Otherwise cropping -1 at a fixed number of bits might be very counterintuitive.
But still, your "simple algorithm" seems too simple. You might easily produce values you won't be able to interpret.
 class ThirdBitMustBeSet(Flags):
 FIRST = 5
 SECOND = 6
 >>> ThirdBitMustBeSet.FIRST | ThirdBitMustBeSet.SECOND
produces error when printing, but otherwise works completely fine. A debugging nightmare. :-/ I'd at least ensure each bit has a separate name if you're going to use that scheme.