test.regrtest outputs to stdout instead of stderr

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-12-30.19:26:59.390>
labels = ['type-bug', 'tests']
title = 'test.regrtest outputs to stdout instead of stderr'
updated_at = <Date 2020-12-30.19:26:59.390>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-12-30.19:26:59.390>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2020-12-30.19:26:59.390>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 42790
keywords = []
message_count = 1.0
messages = ['384084']
nosy_count = 2.0
nosy_names = ['vstinner', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42790'
versions = []