clock_nanosleep() is available in Linux which has POSIX 2001.12 or newer

PEP 7 rules appled for _PyTime_AsTimespec and _PyTime_AsTimeval error checking in pysleep().

In calling clock_nanosleep() EINTR is not stored in errno. Need to use return value of clock_nanosleep()/select() for checking it.

Unix operating systems eg: Linux, macOS, FreeBSD etc. time.sleep() has a resolution of nanoseconds with using clock_nanosleep() or nanosleep() function.

In all Unix systems eg: macOS, FreeBSD, Linux etc, nanosleep() is available.

Waitable timer is 100 nsec resolution. Now, seconds to nanosec conversion is limited in usec, soon will come the next developing part to improve it in next commit.

Waitable timer resolution is 100 nsec but it is limited to 1 usec by round ceiling, moreover sleep for lower then 1 milisec is not possible in Win32 API.

suggested description

Co-authored-by: Victor Stinner <vstinner@python.org>