added acts_like decorator to dataclasses module

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-03-01.14:54:50.430>
labels = ['3.8', 'type-feature', 'library']
title = 'added acts_like decorator to dataclasses module'
updated_at = <Date 2018-03-01.23:20:59.909>
user = 'https://github.com/ninjaaron'

bugs.python.org fields:

activity = <Date 2018-03-01.23:20:59.909>
actor = 'eric.smith'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-03-01.14:54:50.430>
creator = 'ninjaaron'
dependencies = []
files = []
hgrepos = []
issue_num = 32977
keywords = []
message_count = 2.0
messages = ['313096', '313118']
nosy_count = 2.0
nosy_names = ['eric.smith', 'ninjaaron']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32977'
versions = ['Python 3.8']