Use reinterpret_cast and nullptr in pythoncapi_compat.h

[vstinner]

Adjust pythoncapi_compat.h to produce no warnings in C++ compilers.

[oleksiyskononenko]

Thanks, now I don't see any warnings produced by pythoncapi_compat.h. Is this PR still WIP or you're ok it to be merged?

[vstinner]

I was awaiting until the CIs completed for checking for warnings. Thanks for checking for me! The PR is now ready for review ;-)

I updated datatable copy of pythoncapi_compat.h. I already merged the C++ compatibility in pythoncapi_compat ;-)

[vstinner]

Thanks, now I don't see any warnings produced by pythoncapi_compat.h.

That's great :-) I tried many options to avoid emitting warnings when including this file in C++.

I still don't get why some pythoncapi_compat.h functions emit warnings, whereas Python.h doesn't.

It seems like "old-style cast" is triggered when a static function implements a cast inside the static inline function used in C++. I don't understand if it only emits a warning if the function is used or not.