Modernize "Extension types" documentation

BPO	33201
Nosy	@pitrou
PRs	bpo-33201: Modernize "Extension types" doc #6337 [3.7] bpo-33201: Modernize "Extension types" doc (GH-6337) #6411 [3.6] bpo-33201: Modernize "Extension types" doc (GH-6337) #6412

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-04-07.16:36:00.554>
created_at = <Date 2018-04-01.19:03:03.539>
labels = ['3.8', 'type-bug', '3.7', 'docs']
title = 'Modernize "Extension types" documentation'
updated_at = <Date 2018-04-07.16:36:00.553>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2018-04-07.16:36:00.553>
actor = 'pitrou'
assignee = 'docs@python'
closed = True
closed_date = <Date 2018-04-07.16:36:00.554>
closer = 'pitrou'
components = ['Documentation']
creation = <Date 2018-04-01.19:03:03.539>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 33201
keywords = ['patch']
message_count = 4.0
messages = ['314782', '315068', '315069', '315070']
nosy_count = 2.0
nosy_names = ['pitrou', 'docs@python']
pr_nums = ['6337', '6411', '6412']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33201'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']