Rename the posix module to _os

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-05-12.19:16:46.115>
labels = ['type-feature', 'library']
title = 'Rename the posix module to _os'
updated_at = <Date 2016-05-26.20:23:02.386>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2016-05-26.20:23:02.386>
actor = 'ppperry'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-05-12.19:16:46.115>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 27012
keywords = []
message_count = 3.0
messages = ['265421', '265422', '265423']
nosy_count = 2.0
nosy_names = ['brett.cannon', 'r.david.murray']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27012'
versions = ['Python 3.6']