Mock with spec object does not ensure method call signatures

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-06-07.12:04:06.898>
labels = ['3.8', 'type-bug', 'library']
title = 'Mock with spec object does not ensure method call signatures'
updated_at = <Date 2019-11-07.16:48:26.232>
user = 'https://github.com/bclau'

bugs.python.org fields:

activity = <Date 2019-11-07.16:48:26.232>
actor = 'breilly_box'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-06-07.12:04:06.898>
creator = 'cbelu'
dependencies = []
files = []
hgrepos = []
issue_num = 30587
keywords = ['patch']
message_count = 5.0
messages = ['295335', '339269', '339289', '351610', '351617']
nosy_count = 7.0
nosy_names = ['cjw296', 'michael.foord', 'pconnell', 'mariocj89', 'cbelu', 'xtreak', 'breilly_box']
pr_nums = ['1982']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30587'
versions = ['Python 3.8']