how to migrate a c-extension module to one that supports subinerpreters?

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-12-05.19:16:39.226>
labels = ['type-feature', 'docs']
title = 'how to migrate a c-extension module to one that supports subinerpreters?'
updated_at = <Date 2018-12-19.16:23:37.623>
user = 'https://github.com/mattip'

bugs.python.org fields:

activity = <Date 2018-12-19.16:23:37.623>
actor = 'petr.viktorin'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2018-12-05.19:16:39.226>
creator = 'mattip'
dependencies = []
files = []
hgrepos = []
issue_num = 35420
keywords = []
message_count = 2.0
messages = ['331142', '332149']
nosy_count = 5.0
nosy_names = ['jkloth', 'petr.viktorin', 'docs@python', 'eric.snow', 'mattip']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35420'
versions = []