PyPy3 Fixes

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fix "debug_trace_code_generation": use truncating instead of just seeking which is not enough to remove an unused marker as it may not get overwritten afterwards

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extra : transplant_source : J%83%8Di%F7Q%E2%EDk%A1VgO%1A%E1%DE%CDrM%B8

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extra : transplant_source : %A0%CB%0B1_H%F3%CC%D7%2A%A39%E591D%D6i%9F%FE

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extra : transplant_source : %CF%81%20%8A%8D%5B%D8%84%7FE%FFu%88%C5%BF%13.%A3%DD%BE

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extra : transplant_source : z%C2k%EC%3F%0C%A4%BF%CA1cn%2BY/%3EYI%DB%1F

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extra : transplant_source : jsU4%A9%5E%24%81S%10%94%BF%FC%BD%E9-%CA%96%BF%E2

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extra : transplant_source : %15%07%06%F7Tq%91%C6%A4%FA%11%DC%27%E9%0D%D7%FA%A1gx

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extra : transplant_source : 5%FC%EB%8D%0D%CC%F1%7C%CC%909%A9%0C%1C%C2%94Nx%E0m

test: assert that isinf returns -1 for negative infinity

Fixes for buffer protocol docs

[ci skip]

Makes sure (and documents, in a sense) that isinf's declaration as returning
a bint preserves its C semantics.