Stop using internal APIs in _xxsubinterpretersmodule.c.

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-10-15.14:58:34.562>
labels = ['3.11']
title = 'Stop using internal APIs in _xxsubinterpretersmodule.c.'
updated_at = <Date 2021-10-15.18:35:48.170>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2021-10-15.18:35:48.170>
actor = 'FFY00'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-10-15.14:58:34.562>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 45486
keywords = []
message_count = 1.0
messages = ['404027']
nosy_count = 3.0
nosy_names = ['vstinner', 'eric.snow', 'FFY00']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue45486'
versions = ['Python 3.11']