Misc fixes

[skirpichev]

• Update mpz_set_PyLong() signature (return an integer)
• Bump cibuildwheel to v2.21.3
• Update src/pythoncapi_compat.h
• Use PyLong_IsNegative()

Note: first commit taken from #495. I hope that API will be accepted, thus return value for mpz_set_PyLong() will be required. I would prefer include this change here to cleanup #495. @casevh, if you aren't ok with this - I'll remove this commit.

[casevh]

Is there any dependance on PEP 757? If so, should we wait for PEP 757 to be accepted?

[skirpichev]

Is there any dependance on PEP 757?

@casevh, first commit currently is useless. On another hand, it does nothing bad; I think most compilers nowadays will optimize out added branches.

On pros: this will make #495 smaller.

[skirpichev]

BTW, different signature for mpz_set_PyLong() will be required also for PyPy support.

[skirpichev]

Ok. @casevh, first commit was reverted.

[casevh]

Thanks.