RFE: Add os.sched_setattr() and os.sched_getattr() functions

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-12-30.22:59:50.676>
labels = ['3.8', 'type-feature', 'library']
title = 'RFE: Add os.sched_setattr() and os.sched_getattr() functions'
updated_at = <Date 2019-01-05.19:30:27.319>
user = 'https://github.com/mbuesch'

bugs.python.org fields:

activity = <Date 2019-01-05.19:30:27.319>
actor = 'mb_'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-12-30.22:59:50.676>
creator = 'mb_'
dependencies = []
files = []
hgrepos = []
issue_num = 35622
keywords = []
message_count = 2.0
messages = ['332772', '333070']
nosy_count = 2.0
nosy_names = ['vstinner', 'mb_']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35622'
versions = ['Python 3.8']