Issue #22287: On UNIX, _PyTime_gettimeofday() now uses
clock_gettime(CLOCK_REALTIME) if available. As a side effect, Python now
depends on the librt library on Solaris and on Linux (only with glibc older
than 2.17).
Issue #22043: _PyTime_Init() now checks if the system clock works.

Other changes:

* The whole _PyTime API is private (not defined if Py_LIMITED_API is set)
* _PyTime_gettimeofday_info() also returns -1 on error
* Simplify PyTime_gettimeofday(): only use clock_gettime(CLOCK_REALTIME) or
  gettimeofday() on UNIX. Don't fallback to ftime() or time() anymore.
Issue #22043: Simplify time.perf_counter() on Windows

QueryPerformanceFrequency() cannot fail on Windows XP and later according to
its documentation: raise an exception on error and drop the fallback to the
system clock.
Issue #22043: Oops, fix perf_counter() on UNIX if no monotonic clock is
available (unlikely)