Make fields selectively immutable in dataclasses

GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = None
created_at = <Date 2018-12-18.21:13:53.326>
labels = ['3.8', 'type-feature', 'library']
title = 'Make fields selectively immutable in dataclasses'
updated_at = <Date 2019-01-02.00:12:54.319>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2019-01-02.00:12:54.319>
actor = 'remi.lapeyre'
assignee = 'eric.smith'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-12-18.21:13:53.326>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 35527
keywords = []
message_count = 3.0
messages = ['332080', '332086', '332847']
nosy_count = 4.0
nosy_names = ['rhettinger', 'eric.smith', 'yselivanov', 'remi.lapeyre']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35527'
versions = ['Python 3.8']