This PR contains os.sched_* series implemetation.
: os.sched_setscheduler , os.sched_getscheduler, os.sched_setparam and os.sched_getparam.

with this code, RustPython works as below.

>>>>> import os
>>>>> os.SCHED_RR #check os support SCHED_RR policy 
2
>>>>> os.sched_getscheduler(0)
0
>>>>> os.sched_setscheduler(0, os.SCHED_RR, os.sched_param(20))
0
>>>>> os.sched_getscheduler(0)
2
>>>>> os.sched_getparam(0)
posix.sched_param(sched_priority = 20)
>>>>> os.sched_setparam(0, os.sched_param(50))
0
>>>>> os.sched_getparam(0)
posix.sched_param(sched_priority = 50)

notes

• I fixed SchedParam's SlotConstructor implementation, to make sure work as below.
  before : only os.sched_param(1) worked well. os.sched_param(sched_priority = 1) got error.
  after : os.sched_param(sched_priority = 1) is fine

• Inside both sched_setscheduler and sched_setparam, try_to_libc is called to convert RustPython SchedParam class to libc::sched_param. That function can return error(overflow or type error), if SchedParam.sched_priority is not in i32 range.