I have a same problem in 10.7.1. There is no Python API to elevate user's role in a group.
I am working on large script to migrate portal named users to SAML log-ins. ArcGIS API for Python does not seem to have every REST API covered. I ended up making REST calls in some places where Python API doesn't exist or returns error. Here is a code for updating users' role in a group to group admin.
grp_user_role_url = r"{}/sharing/rest/community/groups/{}/updateUsers".format(base_portal_url, group_obj.id)
params = {'admins': user_obj.username,
'token': portal_token,
'f': 'json'}
encoded_params = urlencode(params).encode()
request = Request(grp_user_role_url, encoded_params)
request.add_header('Referer', base_portal_url)
rest_upd_user = urlopen(request).read().decode()
json_response = json.loads(rest_upd_user)