Move captured_stdin(), captured_stdout(), and captured_stderr() to the public API.

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-08.05:12:03.665>
labels = ['type-feature', 'tests', '3.9']
title = 'Move captured_stdin(), captured_stdout(), and captured_stderr() to the public API.'
updated_at = <Date 2020-05-08.22:55:48.097>
user = 'https://github.com/felixxm'

bugs.python.org fields:

activity = <Date 2020-05-08.22:55:48.097>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2020-05-08.05:12:03.665>
creator = 'felixxm'
dependencies = []
files = []
hgrepos = []
issue_num = 40557
keywords = []
message_count = 4.0
messages = ['368419', '368420', '368478', '368481']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'felixxm']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40557'
versions = ['Python 3.9']