[subinterpreters] Explicitly track object ownership (and allocator).

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-05-22.19:19:38.720>
labels = ['expert-subinterpreters', '3.8']
title = '[subinterpreters] Explicitly track object ownership (and allocator).'
updated_at = <Date 2020-05-15.01:51:49.021>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2020-05-15.01:51:49.021>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Subinterpreters']
creation = <Date 2018-05-22.19:19:38.720>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 33607
keywords = []
message_count = 6.0
messages = ['317330', '317339', '317403', '318037', '318091', '368913']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'pitrou', 'vstinner', 'eric.snow', 'emilyemorehouse']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue33607'
versions = ['Python 3.8']