DO NOT MERGE.

This is a work-in-progress/proof-of-concept pull request for not usinglocaltime on Windows for negative epoch values but instead using win32 APIs. We basically have a set of long-standing issues and work-arounds/platform specific code for the fact thatlocaltime on Windows does not support negative numbers, unlike our other platforms.

Some examples from a quick search include:

• Timestamp conversion on windows fails with timestamps close to EPOCH #81708
• datetime.fromtimestamp(t).astimezone() fails for0<=t<86400 in Windows #107078
• datetime.timestamp() fails before 1970-01-02T01:00:00.000000 #94414
• zero-valued timestamps are mishandled by os.stat() in Windows #68134

The changes toPython/pytime.c are the ones to look at, the rest are just enabling tests/removing work-arounds. Note that I had to use anif-guard to only use the new win32 apis when the unix timestamp is>= 0 because as it turns out,mktime on Windows has this behavior where it just doesn't account for DST correctly. Without it, tests that rely on round-trippingmktime andlocaltime fail. (Seethis repo for more details on that)

There is also the question on how to supporttm_yday andtm_isdst. I have an implementation oftm_yday and a commented out one fortm_isdst.

/cc@zooba for thoughts on this approach, whether it's worth pursuing.
/cc@eryksun for Windows expertise on the APIs/methodology

• Issue:Timestamp conversion on windows fails with timestamps close to EPOCH #81708