Deprecate legacy introspection APIs in the inspect module

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-12-01.05:22:05.533>
labels = ['type-feature', '3.7', 'docs']
title = 'Separate out legacy introspection APIs in the inspect docs'
updated_at = <Date 2017-12-01.05:22:05.533>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2017-12-01.05:22:05.533>
actor = 'ncoghlan'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2017-12-01.05:22:05.533>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 32190
keywords = []
message_count = 1.0
messages = ['307362']
nosy_count = 3.0
nosy_names = ['brett.cannon', 'ncoghlan', 'docs@python']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32190'
versions = ['Python 3.6', 'Python 3.7']