Support unittest assertion truncation of repr in error messages

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-07-01.09:27:49.866>
labels = ['3.8', 'type-feature', 'library']
title = 'Support unittest assertion truncation of repr in error messages'
updated_at = <Date 2019-06-03.00:12:40.548>
user = 'https://bugs.python.org/CamillaKe'

bugs.python.org fields:

activity = <Date 2019-06-03.00:12:40.548>
actor = 'gregory.p.smith'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-07-01.09:27:49.866>
creator = 'Camilla Ke'
dependencies = []
files = []
hgrepos = []
issue_num = 27432
keywords = ['patch']
message_count = 6.0
messages = ['269653', '269668', '338217', '341404', '341792', '344343']
nosy_count = 6.0
nosy_names = ['gregory.p.smith', 'r.david.murray', 'Camilla Ke', 'Mariatta', 'cheryl.sabella', 'jiliuk']
pr_nums = ['13091']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27432'
versions = ['Python 3.8']