PEP 670: Convert datetime.h macros to functions

[vstinner]

The datetime.h header file provides two kinds of C API:

• Function pointers exposed via PyDateTime_CAPI structure and a PyCapsule object
• Macros like PyTime_Check() and PyTime_FromTime(): their implementation uses the function pointers

I propose converting these macros to regular functions to avoid leaking implementation details in the ABI. It moves this API closer to the limited C API and the stable ABI (PEP 384).

I propose a first PR to convert macros like PyTime_FromTime() to regular functions.

Later, I also plan to work on a separated PR to convert "GET" macros, like PyDateTime_GET_YEAR(), to static inline functions or regular functions.

Note: currently, datetime.h is not included by Python.h and is excluded from the limited C API.

[vstinner]

I propose a first PR to convert macros like PyTime_FromTime() to regular functions.

Oh, wait. The _datetime extension is built as a shared library, my idea doesn't work... When I built again my local branches, _zoneinfo and _testcapi fail to build.

[vstinner]

I close the issue. I will try to first get PEP 674 accepted ;-) This change is more related to PEP 674 than PEP 670.