mock uses incorrect signature for partial and partialmethod with autospec

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-12-11.14:47:32.441>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'mock uses incorrect signature for partial and partialmethod with autospec'
updated_at = <Date 2018-12-11.14:47:32.441>
user = 'https://github.com/tirkarthi'

bugs.python.org fields:

activity = <Date 2018-12-11.14:47:32.441>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-12-11.14:47:32.441>
creator = 'xtreak'
dependencies = []
files = []
hgrepos = []
issue_num = 35463
keywords = []
message_count = 1.0
messages = ['331631']
nosy_count = 5.0
nosy_names = ['cjw296', 'michael.foord', 'mariocj89', 'pablogsal', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35463'
versions = ['Python 3.7', 'Python 3.8']