I suggest that publishing nanoseconds as a plain int would be a nasty API.  Consider what it would do to os.utime:

    if isinstance(mtime, int):
        # must be st_mtime_ns, it's in nanoseconds, use as-is
        value = mtime
    else:
        assert isinstance(mtime, float)
        # must be st_mtime, it's in seconds, multiply by a billion
        value = mtime * 1000000000

Have we ever published an API that treated a parameter as two wildly different numbers based solely on whether the parameter was an int or a float?