dataclasses: no need for OrderedDict now that dict guarantees to keep insertion order

[ericvsmith]

BPO	32506
Nosy	@ericvsmith
PRs	bpo-32506: Change dataclasses from OrderedDict to plain dict. #5130 bpo-32506: Change dataclasses from OrderedDict to plain dict. #5131

^{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 = 'https://github.com/ericvsmith'
closed_at = <Date 2018-03-21.09:24:43.437>
created_at = <Date 2018-01-06.22:23:30.826>
labels = ['3.7', 'type-bug', 'library']
title = 'dataclasses: no need for OrderedDict now that dict guarantees to keep insertion order'
updated_at = <Date 2018-03-21.09:24:43.436>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2018-03-21.09:24:43.436>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2018-03-21.09:24:43.437>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2018-01-06.22:23:30.826>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 32506
keywords = ['patch']
message_count = 2.0
messages = ['309587', '309634']
nosy_count = 1.0
nosy_names = ['eric.smith']
pr_nums = ['5130', '5131']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32506'
versions = ['Python 3.7']

[ericvsmith]

There are several places where OrderedDict escapes from dataclasses. Switching to dict means we don't have to use OrderedDict forever.

For the 3.6 backport, I'm also going to use dict. I saw an analysis (from Raymond, maybe?) that says there are no 3.6 implementations that don't guarantee insertion order for dict.

[ericvsmith]

New changeset d138892 by Eric V. Smith in branch 'master':
bpo-32506: Change dataclasses from OrderedDict to plain dict. (gh-5131)
d138892