The pitcher is allowed to adjust the ball within the glove. I would regard the flexing of the glove as part of that adjustment. Legal.
You could also consult the COMMENT in 8.05 that says that if the umpire has any doubt as to whether a particular movement is legal, he should consider whether it is intended to deceive the runner(s). In this case, it's hard to see how this could deceive anyone.
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Cheers,
mb
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