[issue21420] Optimize 2 ** n: implement it as 1 << n

[STINNER Victor]

STINNER Victor added the comment:

Oh, the patch implements another micro-optimization: "(x << 0) is x" becomes True. I included it in the same patch to reduce the overhead for 2 ** 0.

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nosy: +josh.rosenberg, pitrou, serhiy.storchaka

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