PyPy: Improve negative overflow error messages in int type conversion

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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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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.