call-matcher breaks if a method is mocked with spec=True

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-08-09.02:43:25.702>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'call-matcher breaks if a method is mocked with spec=True'
updated_at = <Date 2018-12-14.18:11:33.421>
user = 'https://github.com/carljm'

bugs.python.org fields:

activity = <Date 2018-12-14.18:11:33.421>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-08-09.02:43:25.702>
creator = 'carljm'
dependencies = []
files = []
hgrepos = []
issue_num = 27715
keywords = []
message_count = 7.0
messages = ['272209', '272210', '272211', '272212', '286769', '299319', '331846']
nosy_count = 9.0
nosy_names = ['pitrou', 'cjw296', 'carljm', 'michael.foord', 'Eli_B', 'jordan-pittier', 'mariocj89', 'David Hoyes', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27715'
versions = ['Python 3.7', 'Python 3.8']