bpo-34373: Fix time.mktime() on AIX

[vstinner]

• Fix time.mktime() on AIX: fix code checking for mktime() failure
  for years before 1970.
• mktime(): rename variable 'buf' to 'tm'.
• _PyTime_localtime(): avoid abs() which is limited to int type,
  use "localtime" rather than "ctime" in the error message.
• _PyTime_localtime(): EINVAL constant is now always available.
• _PyTime_localtime(): initialize errno to 0 just in case if
  localtime_r() doesn't set errno on error.

https://bugs.python.org/issue34373