pdb: do_debug installs sys.settrace handler when used inside post_mortem

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-21.08:28:00.644>
labels = ['library', '3.9']
title = 'pdb: do_debug installs sys.settrace handler when used inside post_mortem'
updated_at = <Date 2020-11-09.02:50:51.924>
user = 'https://github.com/blueyed'

bugs.python.org fields:

activity = <Date 2020-11-09.02:50:51.924>
actor = 'blueyed'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-03-21.08:28:00.644>
creator = 'blueyed'
dependencies = []
files = []
hgrepos = []
issue_num = 36388
keywords = ['patch']
message_count = 1.0
messages = ['338531']
nosy_count = 1.0
nosy_names = ['blueyed']
pr_nums = ['12479', '23202']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue36388'
versions = ['Python 3.9']