[jorenham]

This fixes the following stubtest errors:

av.sidedata.encparams.VideoBlockParams cannot be subclassed at runtime, but isn't marked with @final in the stub
av.sidedata.encparams.VideoBlockParams is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.sidedata.encparams.VideoBlockParams.__init__ is inconsistent, stub does not have *args parameter "args"
av.sidedata.encparams.VideoEncParams cannot be subclassed at runtime, but isn't marked with @final in the stub

av.sidedata.motionvectors.MotionVector cannot be subclassed at runtime, but isn't marked with @final in the stub
av.sidedata.motionvectors.MotionVector is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.sidedata.motionvectors.MotionVector.__init__ is inconsistent, stub does not have *args parameter "args"
av.sidedata.motionvectors.MotionVectors cannot be subclassed at runtime, but isn't marked with @final in the stub
av.sidedata.motionvectors.MotionVectors is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.sidedata.motionvectors.MotionVectors.__getitem__ is inconsistent, stub parameter "index" should be positional-only (add "/", e.g. "key, /")
av.sidedata.motionvectors.MotionVectors.__init__ is inconsistent, stub does not have *args parameter "args"

av.sidedata.sidedata.SideData is a disjoint base at runtime, but isn't marked with @disjoint_base in the stub
av.sidedata.sidedata.SideData.__init__ is inconsistent, stub does not have *args parameter "args"
av.sidedata.sidedata.SideDataContainer.__getitem__ is inconsistent, stub parameter "key" should be positional-only (add "/", e.g. "key, /")

This also removes a redundant overload of SideDataContainer.__getitem__.