PEP 757: use PyLong_Export

[skirpichev]

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This change export API to Antoine Pitrou -like proposal.

• I'm unsure if we should use signed type for value or store here absolute value.
• Right now, we probably can put compact value to this field, but using PyLong_AsLongAndOverflow first seems to be faster. So, we might just reserve value field for future use.

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📚 Documentation preview 📚: https://pep-previews--3970.org.readthedocs.build/

[vstinner]

LGTM

[vstinner]

This version is maybe not perfect, but it's a great step forward for smaller integers (IMO). I merged your PR, thanks.

I'm not sure if value should be int64_t or uint64_t+negative, we can play with the implementation to tune this.