Way working directory is added to sys.path differs between using -c or -m

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-11-27.10:59:48.432>
labels = ['interpreter-core', 'type-bug', '3.8', '3.9', 'docs']
title = 'Way working directory is added to sys.path differs between using -c or -m'
updated_at = <Date 2020-11-27.20:15:57.198>
user = 'https://github.com/plammens'

bugs.python.org fields:

activity = <Date 2020-11-27.20:15:57.198>
actor = 'plammens'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Interpreter Core']
creation = <Date 2020-11-27.10:59:48.432>
creator = 'plammens'
dependencies = []
files = []
hgrepos = []
issue_num = 42483
keywords = []
message_count = 3.0
messages = ['381942', '381960', '381963']
nosy_count = 3.0
nosy_names = ['vstinner', 'docs@python', 'plammens']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42483'
versions = ['Python 3.8', 'Python 3.9']