better handling of foreign signal handlers in signal.signal

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-23.20:28:37.251>
labels = ['3.7', '3.8', 'type-bug', 'library', '3.9']
title = 'better handling of foreign signal handlers in signal.signal'
updated_at = <Date 2020-01-23.20:28:37.251>
user = 'https://bugs.python.org/StevenGJohnson'

bugs.python.org fields:

activity = <Date 2020-01-23.20:28:37.251>
actor = 'Steven G. Johnson'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-01-23.20:28:37.251>
creator = 'Steven G. Johnson'
dependencies = []
files = []
hgrepos = []
issue_num = 39438
keywords = []
message_count = 1.0
messages = ['360578']
nosy_count = 1.0
nosy_names = ['Steven G. Johnson']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39438'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']