pyclbr.readmodule_ex traversing "import __main__": dies with ValueError: __main__.__spec__ is None / is not set

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-22.17:48:25.398>
labels = ['type-bug', 'library', '3.10']
title = 'pyclbr.readmodule_ex traversing "import __main__": dies with ValueError: __main__.__spec__ is None / is not set'
updated_at = <Date 2021-07-12.17:42:22.974>
user = 'https://github.com/kxrob'

bugs.python.org fields:

activity = <Date 2021-07-12.17:42:22.974>
actor = 'andrei.avk'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-02-22.17:48:25.398>
creator = 'kxrob'
dependencies = []
files = []
hgrepos = []
issue_num = 43299
keywords = ['patch']
message_count = 7.0
messages = ['387520', '397054', '397247', '397279', '397280', '397330', '397342']
nosy_count = 2.0
nosy_names = ['kxrob', 'andrei.avk']
pr_nums = ['24623']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43299'
versions = ['Python 3.10']